Just one language is an absolute no go… first thing, re-memorize the Proto-Indo-European language tree. After that down the Italic and Germanic branches. Frequency dictionary for use with memory towns to get the first 200-300 words down. After that, kick back and watch some tv shows and movies in each language until things start making sense.
After that, it’s time to regain those mental math skills… limited to vedic math, there are some other useful approaches but 80/20 here… you never know when you get struck by lightning again and have to redo the whole thing all over. Maybe also how to solve a Rubik’s cube and how to solve it blindfolded. That and calendar calculation would nicely bridge the gap between mental math and memory techniques.
Should be enough tv by then, so traveling to different countries to actually make some use of the languages… turn that passive vocabulary “active.” If bored along the way, memorize subway networks of the big cities, so that you know where you are. Also, get some new memory palaces as I presume “remember all the memory techniques and lost none of the skills” wouldn’t really include the old palaces, would it?
That would be my approach to get back on track as quick as possible… after that we can discuss Chinese, Hindi (basically, non-Latin script) and other topics…