Consider one day wake up and find yourself on an Alien planet. Now, you don't wanna die and you don't understand their language.
How can you make aliens believe that you are not harmful.
Provided they don't see you as a threat, it's highly unlikely they'll kill you if they have any semblance of intelligence.
There's always the assumption that any alien life-form is harmful, but it remains as just that, an assumption. There's no guarantee they won't kill you, but nothing proves otherwise, either.
There are several factors to take in when faced with such a situation, such as your immediate surroundings, the alien's appearance, nearby (or held) objects, etc...
The aliens themselves could be expecting something from an unrecognizable life-form, so standing still is (most-likely) only going to feed their curiosity.
since you've mentioned an alien "language", let's assume that these aliens have human-like intellect. Then it would depend on what kind of alien you're looking at. If a researcher encounters a non-before seen apparition, they'll want to study it. Make that into a normal person, and they won't want anything to do with it. A hunter would want it as a trophy, a bully would want to instill fear into you, etc.. The possibilities are limitless.
Basically, the answer to this question is, that there is no answer. You're just going to have to run, until the situation changes.
This depends more on the aliens than on you. There are tribes in the rainforest that have killed every explorer that dared to visit them. Others have been very friendly. You might not even have a chance. Think of it like this: If an alien appeared in a rural area where people still believe in demons, the creature would be dead within hours. If it landed in a modern city for example, it might remain unharmed for the rest of its life.
I always recommend people that want to go into first contact or time travel kind of fiction to learn about the exploration of tribes that have no contact with civilization. Explorers also have different techniques they use to establish contact and that no harm is intended.
Sit and do nothing, unless prompted by an Alien. You'll probably die anyway, but that's your best bet for showing that you're not harmful.