So if I understand correctly Oracle is basically complaining about that it’s does not agree that comma is the decimal separator?
From goggling maybe here are hints towards an initial workaround (note NLS_NUMERIC_CHARACTERS):
It’s been a while since I worked with Oracle, but I remember to have had struggles also from Windows clients connecting via JDBC where we had to set NLS_LANG environment variables - a google for https://www.google.dk/search?q=java+sql+client+nls seems to give a lot of Oracle hits.
I don’t think the problem is specific to Metabase, but that it would fail in the same way connecting from any Java/JDBC connection on that specific account/environment settings on that Debian machine. So that’s another thing you can try to isolate the problem.
Hope that can lead you in the right direction.