This script creates new language containing both texts from source and target language in some format, i.e.:
"Some english text <<some translated text>>"
Java8 or higher installed
Unzip, and run with your desired language combination using command line:
poe2-duallang-0.1>bin\poe2-duallang.bat gameDir="D:\Games\Steam\steamapps\common\Pillars of Eternity II" entryLang=de targetLang=en
Wait for it to finish (may take couple minutes). You should see now new language in options.