@kostas294, personally I think you're being generous/reasonable offering the buyer a refund equal to nearly 50% of what he paid.
Had the buyer looked at your photos closely, you can see inside the sealed packaging the figurine comes in and the lettering/verbiage is clearly in another language. I suppose that doesn't guarantee the actual character card enclosed also wasn't in English, but my assumption would have been given the fact the "inserts" inside the package are in another language, most likely everything else inside the package, including the character card, was too.
There was no way for you to determine what language any of the inserts were in without slitting open the package and opening it. In my estimation, buyers, especially collectors, generally like to buy/receive a brand-new item, still sealed in package. Had you opened it, its "value" definitely would have been less.
I suppose the only way to prevent a similar situation in the future is to note in the description that the item is new, still in sealed packaging and because of that you have no way of verifying whether the inserts/character card/sticker etc are printed in English. However, the item is an authentic, official figure by Hasbro, mostly likely produced for the European market, and will come to them new and sealed.
Then, it's up to the prospective buyer to make his/her own decision about they want to do.
As far as whether your problem buyer will win a SNAD case, if he goes that route? Sadly, I wouldn't count on eBay backing you up. It's become almost impossible to get any sort of case decided in your favor and you didn't completely describe the item. Consequently. I hope your buyer will accept your offer of a partial refund. At least all won't be lost.
