I also have a DeepL competitor in my EU country and their hiring practices are similar, rarely hiring, and mostly senior devs and ML PHD grads from the local university. And they're not paying insane salaries, which probably explains why they hire so rarely.
It's probably a business that's difficult to monetize successfully when Google, Microsoft, etc. are offering "free" solutions which are good enough for the casual use, and for professional use when big money is on the line, companies usually pay authorized translators(humans) anyway.
I'm not exaggerating: Kununu.com has €86,000 p.a. as the top-end for a "realistic" software engineering wage in Germany, glassdoor.com lists $84,000 p.a. as the low end for their "most likely" range.
Base salaries are still a bit lower but the kicker is the missing equity