Brazil and Portugal simply do not have the same culture, nor for that matter do Texas and California.

I don't think that has anything to do with the strength of the screening-off at all.

P(T|C&L)=P(A|C) means C screens off L, which does not mean P(T|L&C)=P(T|L) meaning L screens of C. Screening off is not symmetric.

Or in other words, I have not said that if 2 cultures are not the same but their languages are, then the thinking could not be different.

Even stronger separation happens for those who can speak multiple languages, and for these people culture does not screen off language. We can actually "change the language a person speaks" in this case. Do the polylingual talk about being able to think differently in different languages?

Nice one. My prediction is that their thinking would not be affected in any systematic way at the level of abstractions.