Q:

Conditions of Upper-Silesian-Poles minority in Weimar Republic? 

The border between Germany and the reborn Poland in 1918 was decided by the allies.  It was drawn semi-arbitrarily, following the places where the majority of people were Polish and others where the majority were German. 
However, both nations has been mixed for a long while, and there was no clear border in reality.  While other areas (such as Greater Poland) had been annexed by Germany more recently, Silesia was part of the hybrid Germanic world since the late middle ages.  The ethnicity distribution between silesian-poles and germans was very close to 50%/50% so they were subject of a plebiscite vote on march 20th, 1920. 
Because the Soviet-Polish war was on going during the plebiscite, this seriously boosted votes in favour of Germany, as people did not want to be again part of a country involved in a war, and only a small part of Upper Silesia was given to Poland, despite the plebiscite areas being inhabited by a short majority of Poles. 
This resulted in a large silesian-polish minority in the Upper Silesian province of Prussia within Weimar Germany.  My question is : What happened to the Polish populations in Middle/Upper Silesia and East Prussia after they lost the plebiscite? 
The politics of polonophobia and kulturkampf lead by Germany during the 19th century were probably still in memories, despite Germany becoming democratic.  What rights were given to the polish minority that happened to remain within German borders? 
It's hard to find any information about it.  Wikipedia article on Weimar republic says absolutely nothing about the Polish minority and it's condition in neither German, English, or French. 

A:

First of all, it is very important to notice that the situation in both Province of Upper Silesia within Prussia/Germany and The Voivodship of Silesia within Poland was extremely tense, due to the plebiscite and the 3 polish insurrections.  It is absolutely impossible to find a contemporary source which is not extremely biased toward either Germany or Poland, as both nations considered the whole area to be theirs and the other part to be illegally occupied by the other.  Because it was a highly industrialized  region, both countries wanted it more for its economical value than out of care for its people.  This created a very difficult situation. 
My source is "Le Plésbicite et le partage de la Haute-Silésie", Kasimir Smogorzewski, 1931, and it is very pro-polish biased. 
After peace was restored in the end of the 1921, the League of Nations obligated Germany to recognize Poles as a national minority and to have special laws protecting them for the next 15 years. ( On the other hand, the special laws protecting German minority in Poland did not have any date of expiration.)  Approx.  500'000 Poles were living there.  The policy of germanisation in Upper Silesia remained after the war pretty much identical to what it was before.  The area was considered to be definitively part of Germany and the dispute was supposed to be over. 
According to the pro-Polish sources, the laws guaranteeing special minority rights to Poles in German Upper Silesia was simply not applied - Polish was only taught in primary school (and only rarely so), never in secondary school.  Even though there were approximately 5x more Poles living in the Province in Upper Silesia than Germans living in the Voivodship of Silesia, there was more German schools in Poland than Polish schools in Germany. 
People living in Germany were pretty much obligated to have very good knowledge of the German language to have any future.  Also, there was a lot of discrimination from Germans, which was largely exaggerated and used as a propaganda in the Second Polish Republic.  Similarly, Germany constantly claimed that Germans were discriminated in the Voivodship of Silesia, whether that was true or not, and all German parties (not only the NSDAP!)  were revisionist about borders and pursued to annex the Voivodship of Silesia back into Vaterland. 
Interestingly enough, Poles of the area spoke the Silesian Dialect, which differs itself a lot from standard literal Polish language, most notably due to Czech and German borrowings.  Another interesting fact is that Silesia was the only Polish speaking region which was never part of the Polish-Lithuanian Commonwealth, as it had belonged to the Crown of Bohemia and to the Holy Roman Empire before it belonged to Prussia.  The result was that Polish speaking Silesians were not "affected" by the partitions of Poland like the other Poles, because they were already part of Prussia.  Silesia is also the only part of Poland where a significant Protestant minority exists.  All those facts were used by German propaganda to claim the area, and to "proof" that the people living there "weren't really Polish".  Their real considerations, however, were purely economical, as Germans wanted to control the whole industrial region. 
In 1922 in Berlin, an association for Poles in Germany, the Bund der Polen in Deutschland was created ( deutsch english polski ), with the goal to defend and promote Polish culture within the Weimar Republic.  Surprisingly enough, when Hitler took control of the country he did not ban the association, as he feared that this would result on further persecution for Germans in the Second Polish Republic.  They waited as late as August 1939, just one month before the war, to ban the association and imprison its activists in concentration camps. 
German citizens had to serve in the German army no matter if they were Poles or not.  This caused difficulties for them to be recognized as Poles when the Soviet authorities expelled Germans from their homes in Silesia.  Paradoxically, other sources mention the new Soviet regime nourished the "autochthons" and tried to keep as much of them as possible home, despite most knew only dialect and not the standard Polish.  Sources are thus contradictory on this particular aspect.  They however agree that Silesians which stayed in Poland were only partially accepted as "Poles" by their new compatriots. 
Today, the Opole Voivodship remains the last place of Silesia (and of Poland) where the German language is spoken and where many people are bilingual and could pass as either Germans or Poles depending on what the situation would cast.