Subtitle render: Fix an off-by-one error when painting the overlay.

This error introduced in 3244957f4563575d22bf109de27a703558611b19 could cause artifacts to appear when using karaoke effects.

This commit fixes #2773.
Language menu: Ensure "Language" always appears in English in menu entry.

If "Language" doesn't appear in the translated string, "(Language)" will be appended automatically.
Language menu: Use submenus to group language names starting with the same character.
Update translations from Transifex:

 - Greek.