Besides English, what other languages are useful to learn?

Mandarin as China is greatly developing in power, and China is going to overtake the US in economy by the year 2020... It's an economic miracle. Almost 1.5 billion people speak it overall

Japanese is an important language as well, as Japan is the 3rd richest country in the world by economy, and with about 135 million speakers and with many products coming from Japan, it's a very good choice as well, It's extremely easy as there are no exceptions when it comes to grammatical rules.

Spanish is a good choice, but it is harder than Japanese, due to its many different forms of different words, it can get especially confusing. But it is the closest to Latin, French, and English

Hindi (the main Indian language) is possible since India is growing fast in economic power, but the writing seems much more confusing than Japanese and Mandarin (Japanese is easiest out of the two).

Personally I think Japanese, then Mandarin, then Spanish, then Hindi in order of what is best to learn.

I speak Persian and my recommendation for you is Persian. Persian is language of poem , Love and music. Persian currently spoken in Iran (Persia), Afghanistan, Tajikistan, parts of Iraq and Turkey. Persian have 110 million of speakers.Persian is older than many languages and is indo-european language.

Spanish, it's one of the most spoken European language, very useful if you are in Latin America too. :P

FRENCH AND ARABIC