"we are only able to teach something when we really know it. "

I have a thought on this, teaching/explaining something compels us to take an interest and learn more about it. We therefore grow in the language by attempting to teach it. The caveat is that we may end up teaching less than perfectly.

This point was discussed rather extensively on one of my previous OPs:

I don't mind practising languages with fellow learners. Especially when it comes to English we often speak more with other non-native speakers than with native speakers as it is an international lingua franca. When I communicate with other fellow learners of my target languages that have the same level, we get 100% of speaking time in the target language rather than only 50%. Non-native speakers on intermediate or advanced level often know tricks to overcome hurdles in that language and good resources. So, I'm not focused on finding native speakers but either speakers on my level or proficient speakers (no matter if native or non-native).

A native speaker often knows the right way to say but can't explain why it should be that way.

A person who is learning may be of little help as a teacher, but it can be easier to practice with them as they know fewer words and there are good chances they use the same most frequently ones that you know.

So the choice depends a lot on your goals and opportunities. If you have a chance to speak to a native it may be very useful, if you don't you can easily find someone speaking the language you need as a foreign. At least it is always true with respect to English))

Excellent idea, Fernando! That’s thinking outside the box that most language learners seem to be stuck in (judging by the majority of the discussion posts). As Miriam says, when it comes to the international “lingua franca”, practicing with non-natives is a great option, and 100% target language practice is very efficient. I’d just like to point out that Fernando’s written English is excellent — better in some ways than that of many native speakers. Of course, the non-native will on occasion write something weird. In his introduction (good job writing an introduction!), he refers to “people” management, which would be perfectly correct English except that, for some odd reason, we actually use the (more recently borrowed) French word “personnel” in this phrase. It’s no big deal, and continued practice will minimize such so-called errors. Again, Fernando’s written English appears to be for the most part better than some of the native English speaking community tutors’. The challenge would be finding a non-native language partner at that high level. Actually, I’m surprised Fernando isn’t a language teacher already, or is he?