Hello.
Please excuse my long reply, but this issue cropped up during development of the last release and I’d like to get it sorted.
As of v2.5.1, Loco Translate supports DeepL’s formality parameter which can be “more”, “less” or “default”.
I don’t know if this means there are three levels, or if the default simply implies “more” or “less” depending on the language. Perhaps as a DeepL customer, you know the answer to that?
As to what gets sent to DeepL: Loco Translate takes the formality setting directly from the WordPress language code, as per the following examples:
de_DE => default
de_CH => default
de_DE_formal => more
de_CH_informal => less
You can probably see the problem here. What should the formality setting be if no formality is specified? WordPress appear to be inconsistent with DE vs CH. WordPress doesn’t officially support de_DE_informal or de_CH_formal, so does that imply that de_DE is informal by default and de_CH is formal? I can hard-code those rules if so, but I don’t have any clarity on this issue and I am not a German speaker.
Back to your question about how you can set the formality yourself. This is probably something I can add, but right now you would have to set your locale to a code that ends in “informal” to get the behaviour you want. This will probably cause you more problems.