If someone makes too many mistakes in a foreign language, he or she can be difficult to understand, so a reasonable level of correctness is important. However, it is quite unnecessary to speak or write a language perfectly in order to communicate effectively (very few adults in fact achieve a perfect command of another language). Learners should aim to avoid serious mistakes (…) but they should not become obsessed with correctness, or worry every time they make a mistake.
[Michael Swan, Practical English Usage]