I believe I've written about it before, but I'm still admiring London's Free Word Centre's translation residency programme. This time around the resident translators are Ollie Brock (French and Spanish) and Canan Marasligil (Turkish). Ollie is currently collecting untranslatable words, which will feature in the first event he's organising for the residency. You can read more about the plan on the website. Book early for the event starring three poets (ticket price includes a drink!) - and email your word, plus its unique definition, to email@example.com, or tweet @freewordcentre or @Brockollie; or else just contribute it on the night. The best untranslatable words will be mentioned at the event, and discussed in a Free Word podcast.
I am just about to send spießig, which drives me around the bend every time I have to translate it.