The wonderfully disastrous Medium “has no rules against” writing in languages different than English (I know you’ve already understood, at this point…).
Your suggestion may be a very good idea, if Medium was a platform for writers. But one of the drawbacks are tags. All languages are in the same cauldron. If I use 3 tags in English I’m left with 2 tags in Italian, and tags are one of the most important tools for exposure, here. Also, I’m not sure which audience would be addressed, for multilingual posts. I’m not even sure if an audience is addressed now, by the way.
But there are also other drawbacks, so I prefer to handle separate posts. So, I can self-promote Italian posts by mean of a publication which gathers most of my Italian posts. And I get fewer views than a Yeti, hidden in its cave, during a snow storm. Posts in English are in the line of a bear, hidden in its cave, during a snow storm, so it’s not that bad.