The fiddle you posted is working as expected. If you specify a
culture: 'en-GB' option, then OL3 will load the following image:
If you change the value of the
culture option, then different tile URLs are requested:
That answers your question of "Is there a way to set the language of the placenames in the map tiles?". However, if you want to try that for the greek locale (
el), the result is... the default tiles.
Now, if you have a look at https://msdn.microsoft.com/en-us/library/hh441729.aspx , you'll see that MS only offers localization of map tiles ("imagery") for a certain subset of cultures, and greek is not on the list. Unfortunately, you're out of luck.