Steps to reproduce

Create any survey and set the default language to Dutch informal

In my case there was a question about the gender of your child ('Wat is het geslacht van je kind'). The user had the browser setup to auto-translate English to Dutch. I've received a screenshot where the word 'kind' (= child) was interpreted as an English word meaning 'type/sort', so the question translated to the equivalent of 'What is the gender of your sort' (which makes no sense).

Expected result

The question is shown as-is.

Actual result

The question was partially translated.

I believe this was caused by an invalid lang attribute value, it was rendered as <html lang="nl-informal">, which is not a valid value according to https://www.w3schools.com/tags/ref_language_codes.asp. It should just use "nl", for regular Dutch and for Dutch informal.
I suspect there might be other languages that are affected by this issue.