1. BrE marmite: something that people either love or hate, as in ‘Big Brother is television marmite’. After a famous yeast extract spread.
2. AmE redeye: a long-distance overnight airline flight, as in 'I flew to NY on the redeye'. Not all UK dictionaries still mark this as 'North American'. (There's a bit more discussion at the earlier post, where commenters question whether this only applies where there is choice of other times for the flight.)
3. BrE blag: to obtain by trickery/guile, as in 'He blagged a 1st-class ticket'. AmE score is similar, but seems less underhanded. Suggested by @laurelspeth.
4. AmE your mileage may vary: = 'you may have a different experience/opinion'. The abbreviated form YMMV is now known more widely on the internet, but I don't hear it offline and have had to explain it in UK. It comes from the way in which American car manufacturers had to qualify their miles-per-gallon claims in advertisements.
5. BrE plonk: inexpensive (but generally drinkable) wine (i.e. not rotgut). For example, 'you order the pizza and I'll bring the plonk'. Suggested by @AuditorsEditor.
6. AmE to bus (a table): to clear dirty dishes (etc.) from a table at a restaurant. For example, 'please bus your own table'. Also busboy, busgirl: person employed to clear tables at a restaurant/cafe. This is different from 'clear the table' because it can't be used of a table at home. Suggested by @tjathurman
7. BrE horses for courses: means something like 'everyone has different skills, so choose right one for job'.
8. AmE columbusing: explained here. This one may have been premature, since it's a pretty new term, but it is used in circles I belong to.
9. BrE to faff: 'to act unproductively, with elements of dithering and procrastinating'. I find dictionary definitions of this wholly inadequate, and the indecisive element makes it for me rather different from fart around. But there's further discussion at the comments here.
10. AmE leaf-peeping: tourism for the purpose of looking at autumn foliage, done by leaf-peepers. Suggested by @mwnciod.
11. BrE assessment: collective term (but also sometimes a count noun) for any and all work that contributes to the final mark for a course. That is, exams and/or coursework, considered together.
12. AmE to put in face time: I defined it as 'to make an appearance at social/family event for *just* long enough to meet obligation', but others say they use it for business/networking. The face time is the same, but in my experience the verbs are different--I want to get some face time with people I network with, but I have to put in some face time at a grandnephew's christening. Suggested by @Word_chucker.
13. AmE squirrelly: having a kind of nervous dementedness, hence untrustworthy (pronounced 'skwirly'). Suggested by @tonythorne007.
14. BrE throw a wobbly: This is a bit of a cheat, as it's originally Australian--but it is used liberally in the UK. Means something like 'to lose self-control (in anger or panic)'. I suggested freak out as a close AmE relative, but commenter @niblick_iii felt that 'throwing a wobbly has more connotations of being unnecessary or unreasonable than freaking out' (I'd agree). Also suggested by @tonythorne007.
15. AmE weekend warrior: someone who does an activity (especially a strenuous one) only on the weekend (originally used in relation to weekend military training). Suggested by Simon C.
16. BrE sticky wicket: Simon C (suggester) defines it: ‘tricky situation we can get out of if we really concentrate’. Closest AmE is probably in a pickle, but doesn't have that 'we can get out of it' connotation.
17. AmE -grader: e.g. '5th grader'. For child of certain school year. In the
18. AmE klatch or klatsch, particularly coffee klat(s)ch: from German kaffeeklatsch: a group that meets informally for coffee and cake in someone's home. There is a long discussion of whether this is equivalent to BrE coffee morning in the comments at the previously mentioned post. The upshot is: it probably was equivalent in certain settings at one point, but these days coffee morning has a strong whiff of 'charity fundraiser' and may apply to larger events outside the home. Suggested by @SamAreRandom.
19. BrE boffin: essentially egghead with positive connotations. (See this for more discussion.) Suggested by @n0aaa For copious use of it, see the Mitchell and Webb 'Big Talk' sketches.
20. AmE kibitz: (from Yiddish) 'to give unwanted advice (especially to players of a card game)
21. BrE santa’s grotto: a place where kids visit someone dressed as Santa (usually receiving a small gift). Of course, Americans have Santas in shopping malls, and such, but there you 'go see Santa', there's not a universal name for the nook where Santa sits. As a Twitter commenter noted, Americans (of a certain age) are more likely to associate grotto with Hugh Hefner.
22. BrE gubbins: Its individual senses may be translatable, but taken as a whole, it has so many that no one word will do. Here's Merriam-Webster's Unabridged entry for it:
And that is it! The fifth Untranslatable month finished. I'm collecting as if there will be a sixth, but we'll see...