What pisses my off in my 1st language, canadian french, is when someone writes "er" instead of "é" which is the conjugation of a verb.. say "eat" and "eaten" would be "manger" and "mangé" (both sound the same) and people don't bother to write it correctly. No wait, most of them don't even know the differance. I know I'm guilty with my don'tz and doesn'tz but that's deliberate.
Now, since english shares a lot of words with french, I don't even know how to write some words like "differance" or "difference" in both language. One belongs the the other and vice-versa but I'm fucked and I switch from french to english 50 times in a day so it's even more confusing.
Also, I should of thought.
That's actually a big problem for me as well, since a lot of words in english comes from french itself, but are slightly different in their writing, so I fuck up in french easily. I notice it quickly, but so many times I ask myself if there is one or two "n" in the word, or things like you just described.
Also, I think no one who is not french native speaker can imagine how annoying it can be to read someone write "mangé" instead of "manger" or "mangez". Everytime I read it, it irritates the hell out of me because it's just not fucking rocket science to make a distinction between a verb an adjective and an imperative order, and it simply sounds illiterate.
EDIT about japanese : I chose Chinese in my engineer school during the year I had. After one year, we chinese learners could say what time it is, and things like "I like bread. Do you like bread ?". Japanese learners could already plan their trip at the end of the year in Tokio
Apparently past the initial fear of the japanese alphabet, it's actually okay. Chinese is technically very easy (there is no finess in its structure, which is also part of its beauty - because it is pure simplicity and therefore the meaning itself is all of it), but you need the memory of a robot or of a growing kid to absorb the hundreds and thousands of signs with no system of alphabet whatsoever.
French has a tremendous problem IMO, we absolutely never speak like we write, nor like you are taught at school. The oral contractions are frequent, we never use an elaborated tense orally (not even the simple past), we don't use the negation half the time... It's actually terrible, it's also why it is rich, because you can mean so many different things depending on what level of correctness you wanna use (you can imply sarcasm by saying the same thing with a different level of correctness while keeping the sentence itself exactly the same), but the problem is that kids nowadays with internet don't make the distinction. Our generation is the last one which somewhat grew up before computers and keyboards, but I fear for the next one. The resumes must be getting funnier and funnier to read.