This is one of the things that could have been a nice feature, but the devs never fixed it.

It should let you set a default, and options in the list should include ‘detect from browser settings’, and ‘most recently selected’. (Default should be that one probably, but on the very first post it should detect it automatically.)

If they had done that, and encouraged the same in apps and other clients, then filtering by language would actually work and not exclude the vast majority of posts.