Fix DEV_LANGUAGES

If locale/ isn't there or it's empty, it's possible for DEV_LANGUAGES
to have [] as a value. Then LANGUAGE_URL_MAP is an empty dict and the
Prefixer gets all confused and it's clownshoes on fire all the way
down.

This is a problem for contributors working on projects that don't
explicitly set `DEV_LANGUAGES` to something sane in their project
settings file.

r?