I added a very visible, all caps swearword (what can I say ? It just came up ) in all rank names and descriptions for the BasicDiplomatSkill (that is, the displayed name, not the rankname used in the actual code, obviously). Saved, fired the game, new campaign, create diplomat, end turn.

Diplo had the unmodded skill.

The game did not create a new .strings.bin either (but as I said, it's packed)

I thought about something else during the night : I have the French version of the game. Could it be that the cultural versions are hardcoded to only use the packed .strings.bin ? Wouldn't make sense, but it could explain it I guess.