http://www.myglowkeys.com/keyboard.php
I would always go for customization before filling the keyboard with labels that most users won't need and that should be hard to remove. High-quality stickers could even be packed together with the pandora, together with some example pictures, so everyone can decide to start without them, adopt an example-layout or map all buttons himself. That's no big deal...
Give users hard to remove stickers and users will complain that they accidentally put the sticker on the wrong place and it won't wome off.
Give users easy to remove stickers and users will complain about how the stickers keep falling off.
Either way, stickers cause complaints.
-God Ginrai
That's why example-pictures should be available. The idea with registers would still add a lot of comfort to people communicating with people in their own language.
Also, is it really worse if people complain about accidently putting a sticker on the wrong place or if people complain about having to use a knife to rub of the standard key's before they can start customizing their keyboard? Most foreign people will do the later.
There would be a helluva lot more complaints about stickers than the standard setup, trust me.
-God Ginrai