Imo, nobility would make an effort to speak the dialect of their social betters. The planetary nobles would make an effort to speak their betters at the subsector duke's court. It would be like learning a second language, probably, like the working class woman learning proper English in pygmalion. The radioactive rain on ganulph falls mainly on the ash plain.
Nobles of the subsector duke's court who have reason to interact with aristocrats closer to the sector metropole would take refresher courses in the metropole dialect before going there. At home, their dialect would likely be that of the people they spend the most time interacting with. If they only associate with their own social class, they'll speak a dialect close to that of the subsector metropole. If they spend their time working closely with the world's indigenous people, they'll gradually begin to sound like that because of habitually needing to make themselves understood.
Conditions encouraging dialects and linguistic drift include the expense and time of travel, and the lack of immediate interstellar media, and tech levels 7 and lower (no planetary internet).
For imperial organizations, fluency in standard galanglic is probably a requirement for voluntary enlistment.
Imo imperial forces would probably recruit, train and deploy in their local regions to reduce transportation time and cost. If scout courier air filters need to be replaced every 6 weeks, a long distance deployment of major fleet elements would leave thousands of expended air filters in its wake, serenely drifting in space. Ground forces I think would almost certainly deploy from nearby subsectors, or the sector at the farthest.
