Adobe has acknowledged this bug introduced in LR 6.4 / CC 2015.4. You have two options:
- Wait until 6.5 / 2015.5 is released, perhaps in a month or two.
- Roll back to 6.3 / 2015.3: http://www.lightroomqueen.com/how-do-i-roll-back-to-lightroom-2015-1-1-or-lightroom-6-1-1/
In the meantime, even though Adobe has acknowledged the bug, that doesn't mean it will necessarily be prioritized for the next release. Please add your vote and opinion to the bug report in the official Adobe feedback forum: Lightroom: Months in non-English configurations show in English | Photoshop Family Customer Community
Thanks a lot. So I know it's an Adobe problem - and mine, too. I did like you suggested and voted in the official feedback forum.