I'm in a strange position in that I have a dormant langauge I was taught as a child, most of which is buried in my head somewhere, and I have wanted for several years now to get back to it and get up to a reasonable standard (B1/B2). It Doesn't have to be fluent, but I want to be functional, and this is a matter of personal ideal and culture rather than being as "practical" as I can with time and resources.

I work remotely and live abroad. In the next year, I plan to move to France and work from there so I can learn French. My skills here are already around an A2/B1, so I have a good leg up to work with.

In the time leading up to this departure I thought, why not use this time to visit the old language? I'd have about 6 months to rev up my skills in it before going on my French adventure. You could argue that I should be using that time to put my French in as good a position as possible before landing there, but since I'm not a beginner, I figured that's not essential. I have a good degree of freedom and flexibility in my life now and into the next few years, but this rodeo will end eventually so I'm trying to be as smart about this as I can.

My question is, would learning another (albeit familiar) language first interfere with French learning after? Would I jumble my mind as it adapts to a new way of thinking and speaking?