Issue #2509: Adding support for 'locl' in Text Tool

Attempts to resolve issue #2509 (closed) by conditionally adding 'lang' PangoMarkup to gimp_text_layout_apply_tags () in app/text/gimptextlayout.c. According to the Pango docs and examples, applying the Language to the text via the lang attribute will show localized characters if the font supports it.

A demonstration of this patch can be seen here: #2509 (comment 1390777)

Questions for reviewers: Is gimp_text_layout_apply_tags () the right place to put this code? Also, is this the correct workflow or should it behave differently?