Programming language is not just syntax, keywords and standard libraries, but also: processes, best practices and design principles. The latter group I guess is more difficult to learn and harder to forget.

I respectfully completely disagree. not only will you just as easily lose thr processed, best practices and design principles but they will be changing over time (what was best practice when I got my first gig in 1997 is not a best practice today (even just 4-5 years ago not to go all the back to the 90’s)). all that is super easy to both forget and lose unless you live it daily