Problem with some translations?
It's not broken from a different point of view, as VE is not installed here. For performance reasons, we are treating translations in MediaWiki namespace as if they did not exist, unless there is a local site customisation. Otherwise MessageCache would bring the site down.
Therefore showing 3 boxes in https://phabzilla.wmflabs.org/file/data/446pudqx5r4ac3xinuga/PHID-FILE-4mvrbggh4yic6blxbf2b/pasted_file is also expected? Or which font should I install now in order to render those correctly?
I don't know why they are showing to you as boxes, but it is a MediaWiki core change that changed < and > symbols to unicode symbols (e.g. ⧽) to avoid confusing them as html tags. If those appear in the interface, then something is broken, so it should not matter whether they exist in your fonts or not.
@Nike: Please elaborate. What's VE, what do you mean by "local site customisation", and how can we fix the msg-mw template to work around the issue?
VE is visual editor and local site customisations means that a page without a language code exists in the MediaWiki namespace, e.g. MediaWiki:Sidebar.