This is pretty simple:

1) Find the location where the mod is stored on your disk (e.g. "...\steamapps\workshop\content\289070\1655044827")

2) Look for files ending with "_Text.xml".

3) Create new "<Row" entries with the right language code (do not delete existing ones!)

For example this is the description for the "well":

<Row Tag="LOC_BUILDING_WELL_DESCRIPTION" Language="en_US">
<Text>Gives a small boost to [ICON_Food] Food and [ICON_Housing] Housing, but can be a more valuable building if certain early policy cards are used.</Text>
</Row>

If you add these lines directly below, you should see the German text ni your game:
<Row Tag="LOC_BUILDING_WELL_DESCRIPTION" Language="de_DE">
<Text>Verleiht einen kleinen Schub an [ICON_Food] Nahrung und [ICON_Housing] Wohnraum, kann jedoch ein wertvolleres Gebäude sein, wenn bestimmte frühe Politikkarten verwendet werden.</Text>
</Row>

The important part is "de_DE". If someone wants to translate this into a different language, you need to find the right language code. I did not bother to check if also just "de" works so it would also catch different variants of German as used in Austria or Switzerland.