Japanese because Kanji which is still used everywhere doesn't give information on how it's pronounced. Learning to read and especially write them is much more difficult than Spanish spelling rules. Same problem as the language Kanji came from, Chinese. Source: Chinese immigrant who's fluent in speaking Chinese but can't read it. Japanese does have an advantage over Chinese in that it actually has character sets that give pronunciation information though. Presumably you can at least just write in one of the two phonetic character sets and people will understand you okay but I'm not quite sure how the combination of character sets work in practice.