[marcus0x62]

Writing "somewhat freely with the fellow countrymen", while not wrong, is unnatural. Same for "In such kind of language discussions". Those constructs aren't wrong, or even particularly bad, but to me they immediately out the writer as someone who does not have native-level fluency.

[q-big]

How would you formulate these phrases then?

For the phrases that you list, I actually did look them up in the dictionary beforehand when I wrote my comment to be sure that I express them correctly in English.

"fellow countrymen": according to https://www.dict.cc/?s=Landsleute "fellow countrymen" is the English translation of the German word "Landsleute".

If you tell me that in the phrase "In such kind of language discussions" I forgot an "a" (i.e. "In such a kind of language discussions"), you are surely right. Otherwise: according to https://www.dict.cc/?s=solcherart "such a kind" is the English translation of "solcherart".

[marcus0x62]

I wouldn’t use those phrases at all to convey those ideas. Instead of:

“In such kind of language discussions, I often see that in the USA vs Germany, there seems to be a quite different understanding of fluency in the native languages”

I might write:

“When discussing language fluency, I often see that there is a significantly different perspective between Americans and Germans”.

Instead of “In the USA, you are considered to be a fluent (English) speaker when you can talk somewhat freely with the fellow countrymen”

I might write: “In the USA, you are considered to be a fluent English speaker when you can speak comfortably with other Americans”.

There, of course, many different ways to write those sentences. I’m not saying my examples are the best options, or that yours are wrong in the sense of being technically incorrect or incomprehensible. What I am saying is your constructions stick out to me as something a non-native speaker would say or write, and that if I see much of that in written (or spoken) conversation, I do not consider the writer/speaker to be fluent in English. My bar for fluency may be higher than average (apparently even linguists can’t agree on a definition,) but it certainly is not common in the US to consider someone fluent in English based on the sort of limited proficiency that would allow someone to “talk somewhat freely” in English.