I'd misunderstood the role and timing played by the firmware, in terms of when the language indicator got inserted. My mistake.
For what's on right now, it DOES appear to be selecting the English 5.1 audio by default when watching live TV, at least using the Windows app which is all I've tried at the moment. I've tried both 105.1 and 111.1.
I've cleaned out all content from my Recordings folder, to start entirely anew and with no distractions. I've set another recording for that program on 105.1 for tonight so that I will be able to conveniently test all clients whenever I want. I shall look again tonight, to confirm that this latest firmware fix has indeed truly resolved the language issue on recordings as well, on all clients.