Add a Galician translation.

The composition of the translation team can be found on Transifex: https://www.transifex.com/projects/p/mpc-hc/language/gl/members/.
Add a Vietnamese translation.

The composition of the translation team can be found on Transifex: https://www.transifex.com/projects/p/mpc-hc/language/vi/members/.
Track selection: Don't override splitter choice if all tracks have the same priority.

Patch based on Kasper93's work.
Update translations from Transifex:

 - Greek.