In reply to.
That’s how I do it. English for “technical stuff,” Dutch for, say, (inherently more local) news and politics. (I kind of prefer Dutch—I’m simply better at it.) I would love to eventually “cross over” more often, and write about, e.g., the IndieWeb in Dutch, even though no one reads that blog anymore—I would get a handful of readers through Twitter, which I have left sometime last year. ?