Hi there - Here is the extent of my experience on this:
I used to be a soldier in the UK Army. In was stationed in Germany from 1992 to 2006. In that time, I became one of the tiny minority of soldier to become proficient in German, and not through courses or self learning!
I got my foothold in German by going out of barracks with my colleagues and going to the bars where all the other squaddies went. I picked up the essential bar language, then started meeting girls (who all spoke English) and learned a few phrases from them. After a while, I started dating a girls with broken English. That forced my non-existent German to improve a little. I was re-stationed to another part of Germany, and met someone else, this time with no English at all! This further raised my need to communicate more effectively. After that, I reversed the common role of being the one who had to use a second language to talk within a relationship.
Certainly, dating in the target language is useful. That said, whichever is the strongest for both will take precedence. Case in point, I am now in France and my current French partner speaks near perfect English. We almost never speak in French as we need to understand each other more than we need to improve my French.
Ergo, yes dating will help. It helps more if you date someone that doesn't share your language much!