Yeah, I do remember some fluff about Imperial Guard recruits being trained onboard the ships taking them to their first warzone, after being shipped off-world from their regimental muster. The training includes being taught Low Gothic, as well as environmental adjustment to avoid culture shock on deployment (getting Hive worlders used to the concept of 'sky', or teaching feral worlders about technology for example). Which would support the idea that Low Gothic is not anywhere near as universal as it would first appear, but largely confined to the Imperial Organisations who are probably teaching it from the same textbooks they used during the Great Crusade in compliance operations. But if anyone is in the business of regularly communicating with Imperial Organisations, then they would probably learn it too.