Does it really matter a lot when we mix languages ?

Not really. Depends on context. I agree with Zirp in those situations where mixing languages becomes a more of a barrier to understanding, because it defeats the purpose of a language.

But in general sense, I don't think it is a matter of "does it matter". Rather it is something that cannot be fully controlled and it signifies changes in culture.

The sharing of language has been existing for many years. Take a few examples, Korea borrowed words from China and/or vice versa long long time ago. English also has a lot of words originated from other languages. And Japanese has a lot of foreign borrowed words too.

All these naturally occur mainly because people find no expressions in the same language with equivalent meaning or they find that foreign word more trendy.

So I think there is always language mixture with all the migrating and traveling, where different nationalities/ethnic groups mix and learn from each other. Language is ever changing and evolving. And now with the internet, it is happening even more quickly. As long as the language serves its purpose to communicate, it doesn't really matter to mix.

Of course, there will still be people who maintain that a "pure" form of language should be preserved, though you can't be sure how "pure" it really is.

I'm sorry, I didn't understand the question.