The rationale is easiest explained with an example: let
LANG="pt_BR.UTF-8". Tor Browser doesn't have a "pt-BR" or even a "pt"
langpack, but it does have a "pt-PT" package, which we want to
use. Firefox is smart enough that it would pick the "pt-PT" package if
we set the useragent to just "pt", which is what we did before.

However, we also want to modify the langpack we're gonna use (to
change the browser name) and we'll use the guessed locale for
that. Since there's no "pt" package, the previous behaviour will fail,
but it works fine if guess_best_tor_browser_locale() prints the exact
locale for some langpack it thinks we should use, so let's do that.