Here’s how to do it.
While uploading your map in Portal 2, you can only type name and description in English. But after you uploaded the map, you can open it in Steam. You can see the menu on the right. if you don’t see it, press the ≡ button and it will appear. Tap the “Edit name and description” link and select the language you want to add. Then, just replace English name and description with selected language’s. English ones won’t get actually deleted so it can be seen by everyone.
Translation
You can freely translate this to every language. This is already translated to Russian, or what is more correct, this is translated from Russian.