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take German. that way, you can understand when them people take about you behind your back

Prioritise what's important to you. Are you interested in any of the countries those languages are spoken in? Do you know any of the cultures? Is difficulty of language a factor for you? Have you learnt a language before? It might also be a good idea if you check on the syllabuses of those languages. Is it just pure learning of the language or does it come with cultures, history, etc?

In terms of difficulty, Modern Standard Chinese and Japanese are the most difficult, then German, Modern Greek and Italian.

However, note that Modern Standard Chinese is the most spoken language in the world and this can be extremely useful.

At the end of the day, it's good to learn another language.

Italian will probably be the easiest since it is a latin language. English came from German, but the two are quite different and german grammar is hard. Italian is good for the arts and so is german. German is great for philosophy and social studies.

Japanese and Chinese will be extremely hard because they aren't related to English at all. Still, I've taken Chinese courses and it is fun. You're learning will be really slow. The cultures are much different so that might be interesting or a problem depending on your attitude.

Greek? I don't really know. A lot of English words come from greek, especially scientific terms. I'm not sure how hard it is, but it will be extremely useful for ancient history and philosophy. (ok ancient greek and modern greek aren't the same, but modern greek should make classical greek really really easy to learn)

Modern Standard Chinese is for next generation

Standard Chinese. China may be the next powerful country and learning Chinese will help in doing business with them.

If I were you I'd take Standard Chinese or German. I took both because a) Chinese was compulsory at my school and plus I'm Asian, b) I got forced to take German.

choose the country you find most interesting. I personally would choose Japanese :)

take a language that is most usefull in society