Listed below are the languages with the most speakers. If you choose to learn one of these, you will have plenty of people to talk to!
These figures show the approximate total number of speakers for each language, including native and second language speakers. They do not include the numbers of people who have learnt them as foreign languages
It depends on how much time you're able to put into your studies, how often you practise using the language, and the degree to which you are immersed in it.
It is possible to acquire basic conversational fluency, i.e. the ability to understand and participate in ordinary conversations, in 6-12 months or even more quickly if you are immersed in the language and focus on speaking it. To acquire native-like fluency in a language is likely to take longer.
If your aim is to read a new language, you could learn to do so within a few months, if you are able to do plenty of regular study and practise. However acquiring the ability to read the new language as comfortably as your own will probably take quite a while longer. Learning to read Chinese or Japanese takes considerably longer than other languages as there are many more symbols to memorise.
To acquire native-like abilities in understanding, speaking, reading and writing a language, as well as an understanding of the culture of those who speak it, could take anything from five years to a lifetime.