Dont preselect new post language (#1008)

Dont preselect new post language (#1008)

Lemmy-ui currently preselects the first language in the user
settings when creating a new post or comment. This is a bad
idea because this language might not actually be allowed in
the community. It is better to pass the language as None if
the user didnt specify it explicitly, because then the backend
can smartly choose a language based on the overlap of user
languages and community languages.

This fixes the problem described in
[this thread](https://lemmy.ml/post/1066608), where a user
tries to post in a community that has only English allowed,
with all languages enabled in user settings. In this case
lemmy-ui preselects "undetermined language" as default, which
is not allowed and results in an error. This PR fixes the issue
because it lets the backend automatically select the correct
language (English).