""All Guardsmen know Low Gothic and Imperial Codes, while some of the members of the Schola Progenium—such as Commissars and Storm Troopers—may know High Gothic."

This is from the Linguistics skill page.

Does it mean Guardsman specialties or all PCs?

Do all Guardsmen actually start with it? Because Imperial Codes doesn't list it as a starting skill anywhere else.

Tech-Priest should have Techna-Lingua.

Honestly, I don't know what to think of the knowing Imperial codes thing. Personally, I'm inclined to say no, they don't actually start with it/just go by what players end up with at the end of character creation.

This does lead into that you should look at the errata, if you haven't already. PCs do have linguistics skills trained now.

I don't see an issue with giving it to tech priests, most likely an oversight on their part (functionally, its not that significant, it would be assumed tech priests have their own binary language and hand wave the rest).

It's just kinda iffy since it could have easily been another oversight, since I don't think anyone brought up what I cited until now.