In theory, you would think writing little journal/notebook entries and then having them corrected would be a great exercise. Not so much if you're a minority language learner. In fact, once you move out of the major languages, things don't work so well. Here are some of the problems:
1. People who list languages they don't really speak.
Occasionally, it's true, I list languages that I am an advanced learner in as 'fluent', but I do that mainly in the hopes of finding anyone at all to correspond with. But as I browse through the profiles of minority language 'speakers', it's patently obvious that a large percentage of these people don't speak a word of the language, let alone are 'native' or 'fluent'. Latin is particularly a culprit for this.
There's not really a good way to fix this, how do you call people out on this? I suspect the only way to deal with this is in other mechanisms on the site.
2. Writing and correcting entries
Assuming your site has bothered to differentiate enough minority languages instead of lumping them all together, what sort of control is there on what is actually written 'in' those languages. It's no good selecting 'browse Gaelic entries' if most of them are just in English anyway. Introduce some basic crowd-sourced metrics that allow people to vote "not in correct language" on each entry. Enough negative votes, the entry gets flagged/removed and needs to be appealed to be restored. Or at the very least it should move to the bottom of the search results.
Further, implement rating systems for corrections, and make them language distinct. I need to know that learner213 has a great record at correcting English, but has 2 points correcting Greek. And if someone ends up with a woeful negative score for a certain language, consider barring them for 'correcting'.
Third, make it easier to offer corrections in your non-native languages. Yes, it's great to have native speakers fixing things. But native speakers aren't always 'right', and they often don't know why they're right. An advanced/fluent L2 speaker can easily offer corrections, and often explain quite well what exactly needed to be fixed.
All these language exchange websites could be a lot better, to be honest. Most are an exercise in endless frustration.