Add language setting

It's annoying if we must edit ```minetest.conf``` just for change the language, so i add this setting. Although, we still need to restart the game.

Hope this helps!
Not that I'm aware of, but there should be a better way of doing that.

I benchmarked the loop:
```
rounds: 100000
elapsed time: 891.293859
time per round: 0.008912939
```

I imagine it would make menu even slower on weak CPUs (like arm).