Just stumbled across this Reddit thread:
Scroll about half way down the page and you'll find a post by fsultan that details how to change your language of choice - this changes the speech samples AND all the text, including card text and menus, etc, so make sure that you know what the menus and cards do before attempting this or you'll get mighty confused. :) (you also don't want to inadvertently click on the wrong button in the shop and accidentally spend money!).
To do this you only edit a text file.
Scroll down a little more and you'll see all the language options (German, Spanish, French, Italian, Russian, Polish, etc, etc, etc).
Note: I can't imagine that this breaks any terms and conditions but I take no responsibility whatsoever if it does or if you 'break' the game somehow. I'm just posting this link for your interest.
Edit: I see this language change was also mentioned on eu.battle.net in December and none of the mods said it wasn't allowed, etc so it must be fine:
Here's Ragnaros in German:
(wait for the 'Play' button to appear, then click on it).
It's great fun hearing the different voices in different languages. :)
Would be nice if it was possible just to change the speech but leave the text, etc the same.