Offer #1: Well-paid remote job at a startup, cool tech stack where I would learn a lot, but most likely stressful.
Offer #2: Well-paid remote job, irrelevant/outdated tech stack, most likely relaxed environment, and ability to practice and improve German at work.
Context:
I have been in the software development industry for ten years. I've worked for startups, small and larger companies, and bloody enterprises. During the last 5 years, I had technically challenging jobs, with a modern tech stack, and I was also improving my skills and trying to stay in the age. At the same time, I have been learning German as a hobby, reached a B1-ish level, and had a lot of fun learning it and traveling to countries where German is spoken. I would say learning a new language opens a completely new perspective to see a word and, without joking, makes you a different person.
It has become a bit hard to work at my current company because the founder would push us to deliver more and more, faster and of coerce with fewer bugs. Recently he was wondering why we still perform not well enough and told me that he knows people who don't stick nine-to-five, deliver on weekends, etc. He didn't say it meaning forcing me to work more, but more as a sidenote. However, I understood his point well, updated my CV, and got two job offers with salaries 50-70% higher than my current level.
Now I came to the point of deciding what to do next and surprisingly it turned out to be hard.
Normally, I would decline the second job offer or I wouldn't even consider it in the first place. But because of German, I went through the process. They are a small company producing goods. In the team, they speak German, and they wanted to find somebody with whom they wouldn't have to break the routines and have to speak English. They use SVN, Java 8, Vaadin, BPM tools, and some other stuff that is not relevant on the job market. It sounds like serious downshifting. But the more I think about it, the more I understand that it might be a cultural upshifting. There is no doubt my German would be at a new level after a year in the company and this on its own will make me a better person. At the same time, I doubt whether I would be able to do something boring for a long time understanding that whatever I have been doing in my whole career was already more modern than this.
What would you do if you were me?
If they're paying similar amounts, you have non-career benefits (i.e. getting better at German, work culture, etc) and it's a relaxed environment, it definitely sounds worth doing if you're at a point where you want less stress. Sounds to me like you are considering why you're leaving your current job.
Hope this helps :)
Also those places with the cool tech stacks will probably be places with outdated tech stacks in 4-5 years, anyway.
You can also work on a side project a couple hours a week in the relevant tech stack to keep you up to date for when you're ready to jump ship again.
I took a job as a video game producer for a year and a half at a company where I had to speak Japanese (not much, just a few phrases) and sit in on meetings conducted entirely in Japanese, and picked some up along the way (could have done more if I tried a lot harder).
I wasn't programming at all for that job, it was all playtesting game builds, evaluating Japanese games to see if we wanted to bring them to the US, going over Excel files for localization, capturing and editing trailers and gameplay videos with Adobe Premiere and After Effects, designing game covers, websites, and advertisements, handling paperwork for localization, on calls with developers and QA to communicate what we needed, etc. so it definitely wasn't "with a relevant tech stack".
A year and a half later we got some less-than-stellar reviews for a couple games, missed our sales targets, and the company folded, and then I switched back to a technical job programming again. I've since switched tech stacks completely two more times over the past decade (Objective-C -> C#/.Net/Angular -> React/AWS).
So yeah, do the fun cultural job, expand your life in that direction. You don't get that at too many companies. I haven't gotten that at any company I've joined since, they're pretty few and far between (at least those that also pay well).
Also, in my opinion, a stressful startup job is pretty overrated. If it's stressful, it's probably not managed that well, and so it'll probably crash and burn and you won't see anything for your efforts anyway, other than some better skills. Crashing and burning can happen at the fun jobs too (see my experience), but at least you don't have like, PTSD from being overworked at the end of it.
then wait till you get fired at current job, or just keep looking.