Every navigation has a default cache key.
If you're using this bundle, your navigation links may change the url structure after switching the context (browsing from /en to /en-us for example).
Since
/en-us/could be a hardlink, all underlying links need to be transformed (otherwise they would still point to/en/xy)
So, always bind your navigation cache key to the current locale:
{% set menu = pimcore_build_nav(page.document, page.document, null, app.request.locale) %}