DeafBlind (proper capitalization as requested by the community) people absolutely can learn new languages. I have a DeafBlind colleague who is a linguistics professor and knows many languages.

Pro-tactile American Sign Language (PTASL) is growing in awareness and community members. People can do whatever they want, hearing or sighted or both or neither

Braille has different languages

I was born deaf in one ear and have had vision problems all my life. I initially went to college to be a Spanish teacher. I switched majors later, but people can do whatever they want. I don’t identify as DeafBlind but if I tell people about my hearing/vision, their assumptions about my abilities instantly change.

I totally get the mental block of “if you can’t _____ how can you possibly ____ “ but people are great at adapting in a way that works well for them.

Using Braille or tactile sign language, I don't see why a totally DB person couldn't learn a new language. I'm blind and have been learning by using DuoLingo, I don't see why a DB person couldn't do the same with braille if they had someone to teach them the dots for accented letters, assuming they're learning a language that uses them.

I mean ...they learned at least one. Why not more?

It might be more challenging but then ...I am only fully blind , not deaf blind, so I'm not really qualified to say anything about that. I don't know :)

of course. I mean why not?