They may be written in different languages but that’s irrelevant. That’s like saying that Coke and Pepsi are different products made of different ingredients. They are for exactly same use purposes, thus they are competitors. Not equal competitors, though. Discourse is miles ahead in every way and the gap measured in time is getting wider all the time. However, Flarum will gain basic functionalities while the more mature software gains all kinds of minor enhancements and niche features. Thus the gap in terms of software greatness may be even shrinking, depending on what you value.
As an outcome, Discourse will always be more advanced, but Flarum can become a reasonable alternative if you for whatever reason don’t want to use Discourse. As Flarum matures it won’t be significantly worse for many use cases. User experience on both is already very similar because of heavy cross-breading. Yes, Discourse has learned something from Flarum, too. The vertical slider bar on the right when you are reading a topic was initially Flarum’s idea if I’m right. Flarum on the other hand has been watching closely what its big brother is doing which is the main reason for software similarity. And they will continue bringing the best features of Discourse into their software.