[Baeocystin]

>So what do you do? Do you double down on Japanese and try to get some level of fluency and let your dev skill rot?

Honest answer, from someone who grew up as a child of a diplomat (and is also a white, mid-40's middle-aged man)- this isn't even close to an even question. Push your language training as hard as you possibly can.

Linguistic and cultural competency will gain you access to not only the people of the country you live in, but will also gain you the ability to better understand your children as they grow up in a Japanese environment.

Compare that to honing, what, experience with some APIs that will be out of favor in half a decade anyway? There's no path forward if you don't speak the language as it is. Focus on the important stuff (communication) first. Dev skills can be resharpened rapidly as needed.