If you install a language pack as a new language the cache does not refresh and after "importing" the the pack everything stays in English
If you install a language pack as a child of the existing language the cache starts to rebuild and after it the language pack works fine. If you had installed a language pack as a new language too it will now work too
No, if you import a language pack, like Deutsch [Du] for example, as a new language, no cache is rebuilded, only you import the pack as a child of English [US]
I couldn't reproduce that behavior - I imported a language as a child of nothing (a new language) and the rebuild worked correctly. I could only reproduce this when updating a language.