You had better not tell your future Greek In Laws you don't eat meat...
In Poland from year to year situation is better for vegans and vegetarians. In big cities like Warsaw, Gdansk, Poznan, Krakow you can find many vege/vegan bistros, restaurants. In shops(stationary or online) are many products for this kind of diet but still polish people have problems with them. We are traditional meat society and don't understand that someone could not eat meat for any reasons. There is a lot of misunderstanding, malice. I'm vege/vegan and experienced that in the past. Also in some places like restaurans where is serving meat you should be aware that a cook could not understand that a soup on a meat bones aren't vege. Sometimes they think that bones aren't meat (but it is still from animal...).
You can find vegetarian restaurants in every major Polish city. Vegetarianism is going mainstream and there is very little doubt about it.
You can also choose a vegetatian meal on international flights. However, in smaller towns and villages a transition to vegetarianism may appear to be a challenge and a serious change in traditional lifestyle.