What language should my poster for a national convention be?
"The conference language is native language, but english
  contributions are welcome as well.


The conference organizers have made it clear that both native language and English are acceptable and welcome for your conference materials.

That means it’s entirely up to you. If you have strong feelings about supporting your local language, you can feel completely free to use that language for your poster. I am sure many of the native speakers present would find that an agreeable decision and would be somewhat grateful. On the other hand, there will likely be some foreign participants who would find your poster unaccessible and will miss out on learning about your work. Perhaps some of those people are academics you will be interacting with further down the road of your career (e.g., if you end up applying for postdocs overseas).

If you just want your work to reach the largest number of people and don’t care about anything else, I’m guessing English is the way to go (assuming you are in a country such as Germany with a very high proportion of English speakers).

It is a choice to make; neither option is any more right or wrong that the other. 

And if you really can’t decide, how about preparing a bilingual poster?