One good entrepreneur said that focusing first on 2 markets would be ideal when starting out. They went for 5 markets and said it was too much hassle. In the end focused on US/Polish market.

IMO language is big barrier as just testing different text in UI is a lot of hassle when starting out - not to mentions laws, cultures difference, dealing with business partners etc. is much harder due to that.

IMO common law/company to easily hire people across EU would help.