You should copy strings.pak to the data folder containing the original Ultima Underworld files: AppData\LocalLow\Kweepa\UnityUnderground\LooseData\UW\Data
Not all texts will be translated this way, because most of the cutscenes and UI are stored in other game files.
A question for Kweepa: was the latest version perhaps accidentally released with debugging enabled? During gameplay, it creates a Conversations/decompiled folder containing .pseudo.c files.