I speak six languages, one of which, Indonesian, doesn't specifically use gendered pronouns.

I am fairly sure that all human languages give you a way to identify the gender of a person. In Indonesian, the most common way is to say, "the male person", orang laki-laki, but there are specific words for e.g. "girl" , wanita. Gosh, such a pretty language in general.

However, as far as I know, only in English do people ever "state their pronouns up front".

I know for certain that here in the Netherlands, that's not a thing. I'm fairly certain that in France and Germany they don't do this either. Some searching found no examples out of English, and to be honest, nearly all the examples I found were in academic America.

So don't go bringing other languages into it, as they don't do that "state the pronouns" thing at all, to the very best of my knowledge.

Also, your 35% number is off by quite a bit and should be 20% - source: https://www.glaad.org/blog/new-glaad-study-reveals-twenty-percent-millennials-identify-lgbtq