I'd like have an foreign experience in a country of Europe to meet new friends and improve my English. What are the populations that speak correctly that language?
Scotland, Malta... :)
Outside the British Isles, there are only two native English states in Europe, they are Malta and Gibraltar. Malta is a popular place to learn English with a several excellent academies and the advantage of a culture that is very different from the UK and Ireland. Gibraltar is a British territory but does not have many academies, and can be more difficult to find work/apartments because it is so small.
If you're looking for a non-native state, I'd consider the Netherlands and Denmark first, and perhaps Sweden as well.
For English England or Ireland are certainly first choice by a far margin.