Is it true that Hitler invaded Czechoslovakia because the German minorities were mistreated?

Your question contains several implicit assumptions that need picking apart.

Point the first: The hilly areas in the northern and western borders of the interwar Czechoslovakia, which are entirely within Czech Republic, were inhabited by substantial numbers of Germanic-speaking people since the 13th or 14th centuries, at the very latest. Only after 1945 was the German-speaking population of these areas forced to migrate.

Point the second: The term "Sudetenland" did not exist before the early 20th century. Boden says the first occurance in print is 1907. The original term was Sudetendeutsche in contrast to the Alpinedeutsche and the Balkandeutsche, which together comprised the three main geographic and socio-cultural groupings of ethnic Germans within the Austro-Hungarian Empire.

Point the third: Were ethnic Germans in any sense oppressed in interwar Czechoslovakia? Not really. While the new national government in Prague did mandate the teaching of a standardized "Czechoslovak language" in schools (based, of course, on the dialect of Prague), the government of interwar Czechoslovakia did not in any way prohibit of the use of German in public or private affairs. German-speakers (including Jews!) comprised about 23-25% of the population of Czechoslovakia as it existed in the period 1920-1938.

Point the fourth: Did German-speakers in the Sudeten Mountains have some substantial greviances with the new government in Prague? Yes. Did these grievances really have anything to do with ethnic or linguistic issues? No. The fundamental source discontent was that the export-oriented industries in the German-speaking parts of Western Moravia were hurt by disruption to trade routes and markets by the breakup of the Austro-Hungarian empire after the First World War, and then again by the Great Depression in 1929. This provided a fertile ground of high unemployment and general social unrest for the Nazis to exploit starting in 1933, but was largely caused by the global economy undergoing deep recessions in 1919 and again in 1929.

Point the fifth: Germany, under the Nazi Party, starting in 1933, enlisted the help of Bohemian-German politician Konrad Henlein to create the Sudeten German Party (SdP), which was a client and appendage of the Nazi Party in all but name. The SdP had an explicit goal of dissolving the state (Czechoslovakia) of which they were a part. The SdP was dissolved in November of 1938 when it was absorbed wholesale in the apparatus of the Nazi State.

Point the sixth: Did Hitler present himself as the champion of ethnic Germans in Bohemia and use this as an excuse of annex the Sudetenland in September of 1938? Yes. Was this just a ruse to allow him to annex the rest of Czechoslovakia a few months later in April of 1939? Also yes. Were the leaders of France, Britain, and Poland sufficiently, dumb, greedy, and gullible, to take Hitler at his word? Also yes.

To summarize:

Is it true that Hitler invaded and annex Czechoslovakia, bitwise, in 1938 and 1939 on the excuse that he was protecting the interests of the ethnic Germans in those lands? YES.

Was the sufferings of ethnic Germans under the existing Czechoslovak government largely a fabrication of propaganda? YES.

Was it a false flag operation? NO. It was not a false flag, because no outside enemy was blamed. The SdP simply conveniently invited the Germans to invade and occupy a sovereign nation after getting 26% of the total popular vote in the 1938 election.

If you any have additional questions that we can offer clarification upon, please do not hesitate to ask.

Suggested Reading and Sources:

Patrick Crowhurst is the big name in this field and has published two good survey bookd on this time period, in the last few years: A History of Czechoslovakia between the Wars: From Versailles to Hitler’s Invasion, which explains the political history of Czechoslovakia between about 1918 and 1938, with context before and after, and Hitler and Czechoslovakia in World War II: Domination and Retaliation which focuses on the invasion and occupation of Czecholovakia in the period 1939-1945.

Prague in Black by Chad Bryant. This book is mainly focused upon the details of the Nazi occupation of Czechoslovakia, with a particular attention to Nazi nationality policies and their impact on the ethnic Czech populations of Bohemia and Moravia. Does a good job of explaining how the ethnic and linguistic divisions that exist today were not the same ones necessarily operating before the Second World War, let alone the First World War.

Cleansing the Czechoslovak Borderlands by Eagle Glassheim. Exactly what the title would imply. About the population transfers (and outright mass murder) that took place during and shortly after the Second World War, mostly focused on Bohemia and Moravia.

edit: typos, formatting