What effect will the trial of the 'procés' have?
About the legal parameters
Ban Ki Moon, then Secretary-General of the United Nations, said in a 2015 visit to Barcelona: „Catalonia is not one of the areas that have a right to self-determination!“  He referred to the secessionist intentions of the Catalan independence advocates.
There is a wealth of commentary on peoples' right to self-determination, and ultimately Ban Ki Moon's statement can be explained by the fact that the international community recognizes a right to secession only under the following three conditions:
1) Colonial oppressed peoples
2) Militarily occupied areas
3) Where human and population rights are not recognized.
It is obvious that these three conditions are not present in Catalonia. Accordingly, there is also no right of self-determination for the various peoples in the EU, whether Roma, Sinti, Catalans or the Bavarians in Germany.
To my knowledge, all constitutions of the EU member states also contain sections which emphasize the indivisibility of the respective national territory. (Great Britain has no constitution) Finally, the German Federal Constitutional Court has ruled on a constitutional complaint from Bavaria: „In the Federal Republic of Germany, as based on the constitutional power of the German people nation state, the states are not 'masters of the Basic Law'. For secessionist aspirations of individual states, there is therefore no room under the Basic Law. They violate the constitutional order.“
The referendum on the adoption of the Spanish Constitution  in November 1978 had given overwhelming support throughout the country, including Catalonia (around 90% depending on the region), and Article 2 states: „The Constitution is based on indissoluble unity of the Spanish nation, common and indivisible fatherland of all Spaniards, recognizing and guaranteeing the right to autonomy of the nationalities and regions that are part of the nation and to the solidarity between them.“
Of course, the separatists do not intend to allow a secession within an imagined Catalonian state. There you have Tabarnia (the people of the Barcelona region and Tarragona), who announced that they would part with Catalonia if Catalonia separates themselves from Spain), these people were threatened with violence and prohibition, exactly as the Spanish state does with Catalonia.
Legal or legitimate?
There can be little doubt that the failed referendum was illegal. Rebellions of any kind are mostly prohibited. Had the American Revolutionary War failed, the English would have hanged George Washington legally. If Guy Fawkes had succeeded in blowing up the Parliament at the time, the English would probably not set up symbolic funeral pyres on November 5, but celebrate Catholic England together with the Pope. If Stauffenberg had succeeded in illegally assassinating Hitler on July 20, 1944, perhaps all the nations involved in this terrible war would not have had so many deaths.
Accused: the voters of the separatists!
The demonstrations in Catalonia clearly testify that there is a desire for the secession. Although these people are in the minority, but the assumption that their number reaches almost a half, is well founded. What can be made clear to those people with this conviction? Nothing! Opposition to their beliefs only makes them angrier and many who are insecure will be on the side of the separatists in the face of the injustices that will come to light. This was also presumed when estimating the results of the December 2017 elections, where many may have voted for separatist parties because they did not want the central government's police operations.
The 'procés' - a success story!
With the emergence of nations since the 19th century there was also a development for a Catalan nationalism. However, this was characterized by a racist and superior attitude towards Spain and Spainish people. Even at the end of the 19th century, Spanish soldiers in Barcelona received the greatest honors for their war effort against the rebellious Cubans and Filipinos. The Catalan bourgeoisie enjoyed the fact that „the rest of Spain continued to provide massive and cheap labor to serve the only existing industry, the Catalan“ .
After unsuccessful coup attempts during the Spanish Republic and during the civil war, most bourgeois Catalanists went to Franco's side after the end of the Civil War. Already in 1943 Franco decreed that only Barcelona and Valencia are allowed to hold international fairs. A monopoly that was only finished in 1979 after 36 years and finally Madrid was also allowed to hold an international fair (IFEMA). Catalonia received the first motorways, SEAT launched its production in Barcelona. „In 1994, the Government of Felipe González sold Enagás, a de facto monopoly of the gas transport network in Spain, to the Catalan gas company at a price 58% below its book value. Repsol, our only oil company, will also reach Catalan hands.“  notes Rosa Pilar Sáez, a Spanish lawyer on her blog.
Just as the Scottish National Party (SNP) experienced its first peak with the discovery of oil in 1970 (It's Scotlands Oil!), So did the leading Catalanists realize that they no longer would be able to dominate the Spaniards and maybe should have all the richness for themselves.
The „Recatalanization“ strategy paper published in 1980 by the journal „El Periodico“  established the new orientation of the separatists towards independence from Spain and at the same time defined the means by which this goal should be achieved. A masterpiece of social engineering that was to permeate all sectors of the Catalan population, focusing primarily on the language, the media and education policy, without neglecting the penetration of public administration with nationalist people. Recently, the current president of the Catalan government, Quim Torra, affirmed that „the Catalan language is the nerve of the nation“ .
Contrary to the Spanish Constitution and the Catalan Statute of Autonomy, the majority of Spanish-speaking pupils in public schools are largely denied the right to study Spanish. Textbooks, created specifically for Catalonia, falsify the history and build a contrast of Spain vs. Catalonia, a contrast that never existed like this but is intended to outrage the students against Spain and everything Spanish. 
Even small companies that forget that their labels and publications must appear on Catalan, are covered with unfair penalties. Playmobil, although not producing in Catalonia, was recently threatened with a fine of 100,000 Euros. 
Laws of the Spanish central government and regulations of courts, which are not acceptable to the Catalan government, are simply ignored. Karl Jacobi mayor candidate for Barcelona, recommends simply applying the letter of the law in the fight against the violations of laws of the Catalan government 
A study by the Fundación Europea Sociedad y Educación. (European Foundation for Society and Education)  notes that Spanish speakers in Catalan schools are becoming „losers“. In a press conference, co-author Jorge Calero and others stated that the judgments of the Supreme Court must finally be implemented in Catalonia, too. 
The situation of resistance against the 'procés'
The resistance is still unorganized. No help can be expected from the Spanish central government, and it had never really been given. It is not just corruption that has led to constant wheeling and dealing with separatists in all regions, not just only Catalonia. Difficult ratios of votes in the Madrid parliament have often made it possible for minorities to direct the fate of the whole of Spain. This is currently happening, the Catalan government, which does not even represent a majority in Catalonia, refused to approve the Spanish budget and thus paralyzed Madrid.
It is not only for this reason that the great mass of Catalans always felt betrayed by Madrid. The indoctrination of the young people in the schools has led to the conflict partly spreading into families.
The project Tabarnia  (Barcelona and Tarragona), started in 2011 a political satire using all the slogans of the separatists to turn them against themselves, changing „Spain“ to „Catalonia“ and „Catalonia“ to „Tabarnia“. Thus „Spain robs us“ became „Catalonia robs us“. In fact, the regions of Tabarnia are the real economic engines of Catalonia and have a clear majority against separatism. 
In 2017, satire slowly transformed into a political party that was independent of political parties, threatening to secede from Catalonia when Catalonia seceded from Spain. But still this movement is weak and not consistent.
Founded in 2017, the non-party organization „Hablamos Español“ is still weak. It supports the right to use Spanish also in those Autonomous Communities of Spain, where a regional language is the official language. The organization has now been called upon to contact the UN's reporter on minority rights, ethnic conflict prevention, etc.,  as it receives no support from Madrid. It is absurd, Spanish speakers, the overwhelming majority in Spain, must fight for the right that their children be taught Spanish, that they speak Spanish in public offices and that they will not be fined if they run a small bar and give it a Spanish name.
VOX - The Success of Right-Wing Extremism
For the most part, the working class has opposed the 'procés',  they have rather voted for the left. After the PSOE lost massively in the elections in Andalusia and the VOX has come to parliament, it is unclear what will happen in the local elections in Catalonia this coming spring.
There is a certain similarity to politics in Germany. The failure of the left and the parties that call themselves to the left (PSOE)  is a major reason for the success of the far-right VOX.
In addition to other aspects, there are also many supporters for the VOX, because they want to a radical end to the language dictatorship and the separatism. With many other things as represented by VOX, such as radical anti-feminism, homophobia etc, many do not want to have anything to do with it. They only want to „oppose“ the established parties.
Accordingly, half of the PSOE also rebels against the renewed pliability of Sanchez towards the separatists 
The trial is an obstacle to the solution of the Catalonia conflict
In this trial, which is indeed a criminal case, there are inevitably winners and losers. A comparison in which both sides could present themselves as winners, is actually unimaginable. „Right“ or „wrong“ will be decided, and even if it fails to convict any or all defendants for lack of evidence, some will shout „hurray!“ And the other „buuh.“ The declaration of winners and losers, guilty and innocent is the worst conceivable prerequisite for conflict resolution.
The situation in Catalonia today is the way it is because the separatists were left undisturbed and at the height of the conflict only police violence was used. While it was foreseeable that the whole referendum was more of a PR stunt of the separatists, a trap, Madrid fell into it. The back and forth of Carles Puigdemont, when he was supposed to declare independence, testifies to this. It was clear to the leading separatists that secessing from Spain was not a realistic option , they needed and got the pictures of Catalan victims of police brutality coming from Madrid. 
The separatists have managed to make the international media appalled by the sometimes staged incidents and also the publishing of completely absurd casualties numbers. Who else noticed afterward that many media apologized for having fallen for the tricks of the separatists? 
The Spanish central government has behaved as smartly as a bunch of drunken hooligans.
For the coming trial, something similar is to be feared. The Catalan politicians as victims, the Spanish justice as Madrid's puppet. The international press will be fixated on legal battles as at the time of Puigdemont's arrest in Germany.
What is threatened to go completely down, is the common interest of the unionist majority and the separatist minority of the Catalans with the Spaniards. In recent days, the strike of taxi drivers in Madrid and Barcelona was at least in the Spanish papers topic no 1. The international media gave almost no coverage of this. Just as little coverage as the 606,500 employees who lost their jobs at the turn of the year.
In most of the media, the trial will again focus on Spain vs. Catalonia as a contrast, thus forgetting that it is a Catalan conflict.
A conflict that the separatists want to carry into other autonomous communities. In the communities that they consider to be their pan-Catalan empire, such as the Comunidad Valencia and the Balearic Islands.
 Read here the Spanish constitution in English: https://www.boe.es/legislacion/documentos/ConstitucionALEMAN.pdf
 Why Catalonia became rich and Galicia became poor - De cómo Cataluña se volvió rica y Galicia, pobre. https://www.abc.es/espana/20140210/abci-como-cataluna-volvio-rica-201402100444.html#ns_campaign=rrss&ns_mchannel=abc-es&ns_source=fb&ns_linkname=cm-general&ns_fee=0
 English translation: http://catalonia-conflict.eu/StrategyOfRecatalanization.html
 The Spanish Teachers' Union AMES has compared textbooks with editions for Spain and Catalonia, exposing the indoctrination in Catalan schools: http://catalonia-conflict.eu/Indoctrination.html
 „Barcelona tiene que ser la punta de lanza del Estado de Derecho y el cumplimiento de las leyes“. https://cronicaglobal.elespanol.com/politica/karl-jacobi-empresario-aleman-independentismo-alcalde-barcelona_155303_102.html
 Some information in English about Tarbarnia: https://en.wikipedia.org/wiki/Tabarnia , if you speak Spanish, google Tabarnia.
 El cinturón naranja de Barcelona y otros cambios que dejan las elecciones de Cataluña (The Orange Belt of Barcelona and other changes that have resulted in the elections of Catalonia): https://www.abc.es/elecciones/catalanas/abci-cinturon-naranja-barcelona-y-otros-cambios-dejan-elecciones-cataluna-201712221238_noticia.html.
 The letter written by „Hablamos Español“ to the UN rapporteur can be found here in English translation: http://www.katalonienkonflikt.eu/index.php?show=he-brief&idioma=german
 El apoyo a la independencia tiene raíces económicas y de origen social. El apoyo a la independencia tiene raíces económicas y de origen social. https://politica.elpais.com/politica/2017/09/28/ratio/1506601198_808440.html.
 PSOE = Partido Socialista Obrero Español (Socialis Worker's Party of Spain) sister party of Labour
 Crece la rebelión en el PSOE contra la cesión de Sánchez al independentismo: „Profunda vergüenza“ (The rebellion grows in the PSOE against the cession of Sanchez to the independentismo: „Deep shame") https://www.elmundo.es/espana/2019/02/07/5c5b51bbfc6c83c16d8b4653.html
 One of the criteria for international recognition of a secession would be that the secessionists submit a plan in advance of what is following the secession. For example: What about the part of the population that does not want to accept the new citizenship? Bad things happened e.g. with the independence of Lithuania and Latvia, all Russians who do not speak the Baltic languages have been denied citizenship - they are now „sans-papiers“ (stateless people). The presence of the Spanish military, the Civil Guard, the National Police would have to be regulated. If the PR show was perfect, the preparation for a possible independence was below the lowest amateur level.
 A few days after October 1, the counting of the wounded became known. On the eve of the referendum, the Catalan gobernment had given instructions to the hospitals and counted those who saw on television attacks of the police and were afraid: Contamos como agresiones hasta las ansiedades por ver las cargas por televisión.
 for example: Violence in Catalonia needed closer scrutiny in age of fake news
How fake news helped shape the Catalonia independence vote: