Add some helper functions to check if a string matches an ISO 639 code or language.
Play > Subtitles menu: Improve the external subtitles naming.

Show the subtitle filename and detect the language whenever possible.
Sync mplayer.eu.rc.txt after 3c79306a0eca973bcb72061531b680d0d185d809.

Missing string in 8608c253a918f63586d88d3ed7bb285924957163 reappeared here...
Word around AStyle being fool by the non-standard __asm keyword.
Fix some implicit int64 to int32 conversion.