Follow up for issue #1308 (comment),

This need some more investigation as
a) the language combination is wrong,
b) we use from the coding eg. de-AT only the first part for identifying the language, so it should find a fallback,
c) I'm not sure if the warning is from Pronterface, I can't even find any message string for this window. Strange.

Check if the implemented workaround issue #1154 is still needed for macOS builds

@neofelis2X,
can you please add the log file from Pronterface here? Maybe this will show us some more useful information.
For creating the logging file you need to set up a log path in: -->Settings -->Options -->User interface -->Log path
It must be an existing folder where you have full access to. The log file name will be set from Pronterface: Printrun.log