Display subtitle language

This addresses the remainder of [#11715](http://trac.xbmc.org/ticket/11715).

When a subtitle language can't be determined normally, an attempt is made to extract it from the filename. For example:

a.quite.long.filename.for.this.example.English.srt > English [a.quite.long.filename.for.this.example.English.srt]
a.quite.long.filename.for.this.example.Director's Commentary.srt > Director's Commentary [a.quite.long.filename.for.this.example.Director's Commentary.srt]
a.quite.long.filename.for.this.example.srt > Unknown [a.quite.long.filename.for.this.example.srt]