The MoinMoin distribution archive contains the system pages (like RecentChanges) in different languages (see SystemPagesGroup), selecting the correct language in the same way as for the user interface.
For example, if the navi_bar contains a link to FrontPage, moin will first look for a translation of FrontPage into the user's language. So if the user's language is German (de), we obtain the German translation of FrontPage, which is StartSeite. (Internally, moin uses MoinMoin/i18n/de.py for that, which is automatically generated out of de.po,) Moin will use StartSeite if that page actually exists; otherwise, it will fall back to using FrontPage.
This is fine if you want to have a FrontPage and translations of it. But what to do if you don't want that?
Either delete any translated page of FrontPage - they will only be used if they exist. Or simply use the page_front_page configuration variable to choose something different from FrontPage as your front page, e.g. MyProject. Moin will then try to look up a translation for MyProject, but usually won't find any - and thus simply use MyProject no matter what language the user prefers.