A very distinctive part of non-native speakers’ speech is accent. You probably heard a foreign person speaking your native language and struggling to pronounce some sounds correctly. The same occurs to us when we speak a foreign language.
Accent is something that distinguishes native-speakers from non-native. But why do we have an accent? We are born with the ability to pronounce any sound; however, our mother tongue ‘filters off’ the sounds which it doesn’t contain. This means that after 6 years old, our ability to learn other sounds (ones which are absent from the own language) is declining. Thus, we end up having accent in a foreign language in adulthood.
The accent is shaped by the peciliarities of the mother tongue, and the level of accent varies depending on the language we learn. What does this mean? Here is example: I am Russian, and I noticeably will have accent speaking English. This is explained by the fact that the two languages are phonetically distant. There are many sounds which are different. On the opposite, when I speak Portuguese, my pronunciation is close to the level of native-speakers. The phonetics of the Russian and Portuguese languages is very similar. For instance, if you ask a Portuguese person who doesn’t speak any Russian to pronounce a phrase in Russian, s/he will do it easily and you will hardly hear the accent.
To conclude, the later we start with learning a foreign language, the more noticeable accent we will have. However, in some languages the accent will be weaker than in others due to similarities with our mother tongue.