service/set: Correct and simplify behavior related to copying language codes

This corrects cases where it was possible to write more entries into the
write buffer than were requested. Now, we check the size of the buffer
before actually writing into them.

We were also returning the wrong value for
GetAvailableLanguageCodeCount2(). This was previously returning 64, but
only 17 should have been returned. 64 entries is the size of the static
array used in MakeLanguageCode() within the service binary itself, but
isn't the actual total number of language codes present.