In science fiction there are a few well-tried ways to get around this problem, most of them involving hand waving.
- Have some sort of universal translation device e.g. the Babel Fish
- Have the traveller live in the alien culture and learn the language over an extended period of time e.g. Out of the Silent Planet
- Have a reason why the language is like a dead earth language and take along an expert in that language e.g. Stargate
- Have the aliens intercept our radio and tv signals and decode them, despite the fact that they aren't even compatible worldwide e.g. The Simpsons
- Use maths and chemistry to start the communication. I'm not sure if you can get to war and peace like this.
- Ignore the problem, have everyone speak English and hope that no one notices e.g. Star Trek
I have to admit that this last one is tempting although I find it unsatisfying.
If you were writing about someone meeting a totally new group of sentient beings, how would you get them to communicate?