I wonder if 39 or any number between 31 and 38
might have been at one point considered as a base game roster, or something close to a rudimentary selection of civs the same way I suspect that Poland was intended to be base game Civ VI at some point, as Jadwiga was on the civet chart, winged hussars were in the release trailer and Polish soundtrack and color jersey made its way to the base game files. Now we may have seen civ selection screen from a build that has placeholders for DLC civs and we may have seen a leader that fits better with a DLC civ than a base game one. If so, the upside is that the devs may have to crunch less upon the incredibly complicated civ designs and art, so I intend not to moan about the DLC monetisation schemes at all. (ironically, this post has also been cut up into two for the purpose of two different threads)
Now if we weren't there before, and because I'm currently very bored, now we're entering crackpot theory territory: I propose that neither of Britain nor Germany may be base game. A piece of evidence I put forward is the Norman civ selection screen, which most likely would have the last European civ on it obscured. To find out which one fits the most, I found the font used on the screenshot (most likely Core Sans AR 45) and recreated every piece of text on it as closely as I could. I assumed that the HIDDEN boxes obscured pretty much exactly as much as they had to, because it matches perfectly with Charlemagne's name. Red is for names that don't fit, green is for names that do:
America fits perfectly (its perfect down to the last pixel - there are 4 pixels between the end of "Charlemagne" and the end of the HIDDEN box in my measurements, and there are 4 between the beginning of the second HIDDEN box and the start of hypothetical "America"), Britain a little bit is too short, Prussia is close, Russia wouldn't be there anyway and Germany has no chance to fit. Now, I have been wrong more than once about civ selection screens, so take it with an abnormally big grain of salt. To summarise, I don't know if we've learned anything from my experiment. Maybe except that we probably aren't getting another Norman leader.