Make sure that the new caching mechanism for flattened labels respects languages.

Make sure that the new caching mechanism for flattened labels respects languages.

 * Counters.cpp (flatLabelString): return a cache of the flattened strings for each used language
 * Counters.cpp (theCounter, counterLabel, flattenLabelString): add a lang parameter; in theCounter, populate the cache as needed.

 * insets/InsetCaption.cpp:
 * insets/InsetFoot.cpp:
 * insets/InsetBibitem.cpp:
 * insets/InsetCollapsable.cpp:
 * Paragraph.cpp:
 * Buffer.cpp: pass a language argument to counter methods.

 * Paragraph.cpp (translateIfPossible): use the function with same name in gettext.cpp.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@30520 a592a061-630c-0410-9148-cb99ea01b6c8