mkvmerge: OGM reader: fix handling chapters encoded in non-system charset

OGM comments might be written in a charset other than UTF-8. During
identification we simply want to know how many chapter entries there
are. As mmg's users have no way to specify the chapter charset when
adding a file such charset issues may cause mkvmerge to abort or not
to show any chapters at all (if "parse_chapters()" fails). So just
remove all chars > 127.

Fixes #919.
translations: update list of translatables & German translation update