I discovered it accidentally while browsing on their website. Check this link to have a look: https://www.bitstamp.net/article/help-translating-bitstamp-you-local-language/
So what to do? Some of the language are completely lacking translation (like Maltese, Icelandic or Afrikaans).
So your job is to translate, with an online translator or wiki, the LIST OF THE COUNTRY'S NAME that every language has: you'll find it easy and you'll won't need a knowledge of the foreign language.
I did it with Lithuanian and Greek, without knowing any single word of them, and i got more than 1 month of 0% bonus. Check it here: http://imgur.com/8aOw5In
If you found that useful, and i made you save a lot of money please consider a small donation: 1MYHxTNMBiJujRMpHPPfEetx7T26xdw9Gi
enjoy!