Pintolus

Any idea on how to do it? This is really important to me because otherwise i cant use the plugin.

Thank you so much for your answer!
Yes SEO seems to be an issue, since the content will be duplicated – could even lead to a google-ban.

but it is my understanding that WordPress, itself, can only run in one selected language at a time

I thought this too, but just 5 Minutes ago i’ve read about ‘WPLANG’ which is defined in the wp-config.php. This may look constant, but it seems that you can also set it up, that it changes for every individual user using sessions.
Source:https://code.tutsplus.com/articles/new-wp-config-tweaks-you-probably-dont-know–wp-35396 => Nr. 6

I will try this out tomorrow, but first i have to translate my theme and some plugins.