Change submit button text in contact form

I’m using the Otter Blocks contact form on a German website, but I don’t find a way to translate the submit button text. Any advice how to do this?

There is another thread from one year ago asking the same question, but all the solutions mentioned there don’t work with the current version.

This is such a fundamental feature for every site owner outside the English-speaking world that I wonder if there is an obvious solution that I just don’t see.