Bring down languagecache size if more than one language is configured

Bring down languagecache size if more than one language is configured

We do not care about most of the pre-defined ids, so using a cache
with the size nlang*ID_NUM_INTERNAL uses way to much space.

Rewrite the cache code to use a cache of the size ID_NUM_INTERNAL
plus (nlang+1) times the used keynames. Optimize the nlang=1
case so that it also uses less memory.