The new OCR action and workflow from a screenshot is really cool. It would be great if there was a new action that would allow us to translate the output text using Apple's built-in API too (I's rather not use GPT etc.)

I think the "Translate Text" Action in Apple Shortcuts uses Apple's built-in or native API. You could create a BTT Action that calls an Apple Shortcut that you create that translates the input text into your desired language.