A company in Great Britain put together a list of the world's most difficult-to-translate words. Here (via the BRI), are the top 10 English words voted the most difficult to translate into another language:
10. Kitsch "An item, usually of poor quality, that appeals to common, or lowbrow tastes"
09. Chuffed (from the word chuff) "puffed with fat" but now meaning "proud, satisfied or pleased" in British slang
08. Bumf (short for bumfodder) "toilet paper" but in British slang, often refers to "paperwork in general"
07. Whimsy "a quaint or fanciful quality"
06. Spam - as in the luncheon mean, not the junk mail variety
05. Googly (from Cricket terminology) "an off-breaking ball with an apparent leg-break action on the part of the bowler" (Although I don't think this term makes much sense in English to begin with)
04. Poppycock "nonsense; empty writing or talk" from the Dutch pappekak, which translates literally as, "soft dung"
03. Serendipity "finding valuable, useful or pleasant things that you haven't been searching for; happy accidents"
02. Gobbledygook "wordy; unintelligible nonsense" (Oddly enough, Blogger and Firefox's auto-Spell Check flagged several words on this list, but not this one)
01. Plenipotentiary "a special ambassador or envoy; invested with full powers to negotiate or transact business"