Diaspora/Diaspora

what is already taken?

"Username". I'm not sure if we actually show this one to the user.

If it's not in the UI, then we shouldn't really offer it up for l10n. Google ended up with a massive headache because they just kept unused legacy strings.
If it's still in use, this is a bad approach because it forces English Subject-Verb-Object syntax onto translations i.e. it's a big problem if your language does not start sentences with a Subject (of which there are many). If there is a placeholder, then it should be over i.e. if we offer
%s is already taken
that would be better because it can be re-ordered as (in our case)
Tha %s 'ga chleachdadh mu thràth

Yes we do need to cleanup unused translations. It's a huge effort to do so and nobody was willing to spend the necessary work yet.

We try to take care and use interpolation where possible. Somebody has to check if upstream (Rails) has done something about this case, but we don't control the caller it.