Did you try if running Lazarus with sudo makes any difference?
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Following Thaddy's script (elevating group rights) made Lazarus 4.0 IDE unusable. Removed previous installation, then installed a new one as described in "README.md". It works (an error of recompiling IDE persists anyway). Normal usage does not show problems at first sight.
The difference from previous installation may come from system updating (it is quite new version Linux Mint Mate 22.1 Xia and a number of daily updates is quite high).
About the MessageBox ...
It seems to me that IDE uses its own message box and it allows to control how it is localized.
In trunk it follows IDE setting (localized to a chosen language which may be not that in OS).