Packingsoapandwater:
In reality, it's extremely difficult to learn another language to near fluency post-18 while dealing with everyday working life, and build a sustainable life in a non-tourist area in another EU country .
This may be true for those who did not learn other languages to near fluency pre-18. Learning languages seems to get easier once you have learned a few while young. So I have noticed, anyway.
All my children learned several languages pre-18. And all of them, at various times, built sustainable lives in non-tourist areas in other EU countries than those whose languages they already spoke. (Not Germany, interestingly given your mention, Packing .) Some have returned to UK, but others remain elsewhere in (non-tourist areas) of EU, all with children and partners of various mixed nationalities -- and all with well-paid responsible STEM jobs, some public, some private sector.
It is possible, then. One of the things that puts the grit in the machine of Brit multilingualism is the abysmal standard of school language teaching and learning in UK (particularly England, I have to say). Those of my children who settled back here in England are horrified at what passes for teaching of languages to their children here, so much so they seriously speak of leaving again just for this reason .
And, again, of course, this is all subsumed in the strange British exceptionalism that accepts mediocrity and worse while claiming its superiority, and which was part of what fuelled Brexit.
What goes around ... Now, of course, post Brexit we have less chance of putting anything like this right. But we can rejoice in the maintenance of our insular monolinguism as we glory in our xenophobia.