TL;DR: Humanity Wears Many Hats

[Haighus]

Thinking on it if High/Low Gothic is the official language of the ecclesiarchy then everyone is probably automatically obliged to learn it.

This isn't borne out in the background though, and that makes sense because it is impractical to force every feudal or feral or frontier world to learn Low Gothic. The Ecclesiarchy (despite their best efforts) isn't universal, and many, many other sanctioned sects of the Emperor's worship exist. As already pointed out, when a planet is colonised, rather than outright destroy the existing religion, they nudge it to become a version of the Emperor-worship. Things like 'Well, this god you are worshipping, you see, he is actually the same as the Emperor, so you should just start calling him that...'. Very much like how real world religions have done it- Christianity would have been a much harder sell to pagan Europe if they didn't do things like take the pagan Midwinter festival and turn it into Christmas (and be damned when Jesus was actually born in the year). Only the larger, important planets have a proper Ecclesiarchy presence.

As it stands, background stuff tells us that Low Gothic isn't universal- Fenris speaks fenrisian, Tallarn speaks it's own languages, even Armageddon clearly has a heavy distinction between dialects from the Primus and Secondus continents, as shown by Helreach.

[CoffeeGrunt]

Plus it's worth noting that the whole point of the Imperium is that it's an utterly failed homogeneity. The Ecclesiarchy never stops burning heretics, the Guard never stops putting down rebellions and the High Lords of Terra never stop fighting for those thrice-damned tithes.

So Yorkie's point is pretty fair: it's impossible to expect the entirety of the Imperium to learn Low Gothic, and in practice, it almost definitely doesn't because the lingua franca is just one more aspect of the Imperium's failure to keep Mankind united against the coming storm(s.)

[Path Walker]

Yeah, I have always thought of Low Gothic as an official language that most citizens or subjects of the Imperium don't speak, only those with official position or dealing with the wider imperium would speak Low Gothic, so its like a trade language that can be used to smooth over conversations with people from around the galaxy.

The local language might be rooted in Gothic (low or high) or might have had a different root, being the ancient language of the colony has spoken since Old Night or influenced by any range of factors.

On joining the guard, one of the most important parts of the training is in Low Gothic, to enable Commisars and Generals of different regiments to issue orders to guardsmen and have it understood. I'd imagine even then, there is some variance across warzones, and individual guardsmen might not get to grips with it and rely on squadmates to interpret things for them.

[CoffeeGrunt]

Hmmm, perhaps that's part of what issuing Orders in the Guard Codex represents, with higher-ranked Officers being more able to discern, "aim for the fething thing's eye socket," and thus act accordingly, whereas lower-ranked and thus lower-trained troops are like, "Was war das, Kamerad?" The Leadership represents that, to a degree.

Hmm, given that I fluff my Guard as being drawn from a local series of systems under an Empire, I quite like the idea of that being the reason why my Guard instead are pitifully firing their Lasguns at Carnifexes' toes all the time.

[Haighus]

It also would work quite nicely with the 5th Edition Cadian hero who could issue orders to his own Veteran squad- being the poster boy of the Cadian military, he would be pretty well educated and trained very well, and his squad likewise would have that education likely.

[Path Walker]

I'd picture a highly militarized world like Cadia, where service is a given and life is ordered and regimented around joining the Guard, that Low Gothic was the only language spoken, as they wouldn't want to waste time teaching a language they won't use in war

[Haighus]

True that. Good point.

[Gotthammer]

Many hats