Travel by train in Europe

Last summer, I travelled by train in Europe with friends. It was a great adventure.
We went to Italia, Croatia, Greece, Slovenia...

I'd say 'I travelled around Europe by train with friends. 'Around' gives the sense that it's for fun. So for the title I might say 'Travelling around Europe by train'. We also talk about 'interrailing' in (UK) English for this activity, because the train ticket is called an Interrail pass, at least in English.

In Italia Italy, we visited a lot of museums, including the Vatican Museum 'including' is less formal than 'such as', and you need an article. In other countries, It was more festive especially in Crotia I might say 'other countries had more of a festival atmosphere'. Parties are really great. We met a lot of foreigners from all around the world. Instead of language barrier, we felt the approach with others was much easier than in France. This makes sense but isn't idiomatic. I'd say 'Instead of [or rather than] being inhibited by language barriers, we found it much easier to approach others than in France.'

My favorite country was Slovenia. People were so nice...and the landscape was just amazing. I swam in the well-known bled lake Lake Bled.

The return to Paris was difficult even if I love this place... I liked that felling feeling of moving being on the move OR moving from place to place.

I hope this isn't too late, and that it helps! The corrections are mainly just making your English more idiomatic, rather than because there are rules you aren't following.