Writing style: require US spelling, and maybe British punctuation?
Currently the HIG have a section on writing style. I'd like it to add a mention that strings should use US spelling. This rule is obvious to most developers because we have a separate locale en_GB for British translations ("colour", "wastebasket", etc.), but British spellings sneak into some apps by accident on occasion, and it would be good to, er, spell it out explicitly in the guidelines.
There is another elephant in the room: punctuation. We currently have no punctuation rules for GNOME apps, so I would naturally assume that following normal US punctuation (punctuation inside quotes, "like this.") is a good default choice, but the documentation team actually has a rule to use British punctuation (punctuation outside quotes, "like this".), and I see the HIG's writing style page itself follows that and uses British punctuation. So honestly, after years of working on GNOME, I'm still not sure which style to use in our UIs. One could argue that following the default locale as for spelling makes the most sense, but the British style is more clear, and is adopted by the text of the HIG itself. I suggest we explicitly recommend one way or the other so that application developers have some clear guidance on this.