It is because in order to localise landed titles in Crusader Kings games (both 2 and unfortunately also 3) you cannot do it through normal localisation files and instead have to edit the landed_titles file which stores the whole de-jure titles hierarchy and most properties of a title (such as creation rules, culture, colour, etc).
Since modifications to that file can be considered cheating, it makes sense to disable achievements for mods that edit it. It is however a real shame when all your mod does is basically just visual flavour changes and leaves everything else intact.