lib/bg.po,lib/de_CH.po,lib/id.po: Translation updates.
src/filename.c: Comment tweak.
src/mainfrm.cc: Fix measuring line to show change in altitude rather
than altitude (broken in 1.2.11).  Reported by Brian Clipstone.
lib/de.po,lib/de_CH.po: Translation updates.
src/osdepend.c: Don't use isalpha() for checking drive letters as
it's locale dependent.