The main thing is to work out which language you want to learn. If you’re going to be passing through a whole bunch of countries then there’s no way you’re going to learn each and every language. Instead, you can try to focus on the most important ones which may well be understood in neighbouring countries or states. For instance, whilst there are 15 national languages in India, the influence of Hindi is so huge that it allows you to get by in most parts of the country. Thank Bollywood for that one.
Whilst learning something like Spanish will serve you well in various countries, learning a minor language will often endear you to the locals even more. If you learnt Vietnamese, for example, the locals are likely to be completely stunned and take you utterly to heart. It just might not be much good to you once you leave.
Languages come in families and, as English is something of a composite between Saxon and Latin, the Germanic and Latin languages are by far the easiest place to start. There will be many words such as informacion in French that are instantly recognizable and there’s unlikely to be huge problems with pronunciation. Trying to get the throat rasps necessary in Arabic or nasal sounds of Hindi might prove tougher.
Even harder are the languages that have their own alphabet. That means unless you can find a transliterated dictionary that spells the words phonetically, you either have to learn each new word direct from someone’s mouth or else learn to read all over again. It can take months until the flow of an alphabet like Hebrew makes much sense to you, never mind something like Mandarin, where every word has its own picture to memorize.
Remember that when learning a language, you’re not preparing for an exam in school. No one will fail you for making mistakes and the only real degree of success is how much enjoyment you get out of your communication. Ultimately, you could speak all the languages in the world and still have nothing to say.