Rename LangString::all_false() to default()

[jyn514]

LangString::all_false is not actually all_false - rust will return true. It would be nice to rename it to default() at some point (and maybe add impl Default for LangString while we're at it).

cc @poliorcetics

Originally posted by @jyn514 in #80013 (comment)

[poliorcetics]

@rustbot claim

I'll either get to it before next week is over or when #79454 is merged, whichever comes first