> absurdities: Genders for non-gendered things is one example.
This. I never understood how e.g. Spanish speakers think that a door is female or a clock is male. I mean, it's not like there are any body parts you can examine for a definitive answer, or clothing and mannerisms which let you make a pretty good guess...I never really got a satisfactory answer other than "it's usually -o or -a, but not always; really, you just have to memorize it." Seriously...WTF?
> diacritical marks
Other European languages love them. To me as an English speaker, they look like misplaced inkspots or dirt on my monitor. I never had any class in school or college that taught what they mean [1]. I blithely type "fiancee," "naive" and "Geiger-Muller," since I don't want to get out Character Map or whatever the Linux equivalent is [2], and I'm not really sure which marks to use or where to put them. I pretty much pretend they don't exist, unless they cause compiler errors [3], in which case I terminate them with extreme prejudice.
[1] I did once learn that an overline (a line above a character; I don't know if that's actually what it's called) means a long vowel sound, and an upside-down e means schwa. I haven't seen either of these used outside dictionary pronunciation keys.
[2] In my current operating system, Linux Mint, I don't even know how to get those characters other than copy-and-pasting the Unicode text somebody else has put on a webpage, or spending an hour or two sitting down with the RFC's that specify UTF-8 encoding and a hex editor. The only reason I know on Windows is that I eventually stumbled on Character Map by curiously exploring all the menus. This may give you a clue how often I deal with international text
[3] http://news.ycombinator.com/item?id=5316875