That sounds like a possible bug in the site code. Testing the Moments page in other languages looks like it applies the language settings to the main page elements (header, footer, etc.) but not to the Moments themselves. Here are some things to try:
1. Try a different client
If you have a smart phone or tablet, try the Android or iOS Twitter app -- both of which often behave differently than the web app -- to see if they show the Moments in the correct language.
2. Try to contact Twitter support
If you can get a hold of someone on the Twitter support staff, they may be able to help. Two options are through Twitter via @TwitterSupport or by email at [email protected].
If neither of those options get a response from a real person, you could also try tweeting @TwitterMoments, which may or may not get human attention.
3. Submit a ticket
You can submit a problem ticket through Twitter's Help Center. Though this may eventually result in the problem being fixed, you probably won't receive a response.
From the Help Center page, select Profile. When asked to select a reason for your ticket, choose other.