Only parse the locales ones

Before we'd go over the entire list for each translation (so each app)
we'd use translation for. This means we'd also go over thise locale list
(currently containing 750 entries) * apps so often this added up to ~20k
calls.

now we just dump the locales in a set once and then check if the entry
is there.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>