Hi Ben,
your browser is apparently set to the content language "de-DE". This is why UI5 attempts to load the respective language file. (containing translatable texts in UI5 controls, but the same mechanism is also used for translatable application texts)
UI5 is translated to 37 languages, but there is only "de", not "de-DE", "de-AT" etc. So the fallback mechanism should kick in and "messagebundle_de.properties" should be loaded properly. (In case of not-available languages it would fall back to English.)
Does this happen?
So basically the 404 is expected and no reason to worry (except that it adds a useless request, of course...).
To avoid the useless request, at least on Java servers (not sure about the ABAP implementation) the optional server-side component of UI5 (the resource handler) will do this fallback logic on the server and directly serve the best suitable file even if a non-existing laguage is requested.
Does this clarify what's happening?
Regards
Andreas