Could you please change this...? I don't want German names for the Italian, Bohemian or Dutch titles, only because the emperor is German... Why should the Duke of Brabant call his city Löwen instead of Leuven if the Emperor is German? For titles the local culture should be more important then the culture of the realm. The same with Occitan titles. Or Russians! They wouldn't use Mongolian names, only because they became vasalls of the Golden Horde.
Been thinking about this, not sure yet what the best solution is.
I agree that a Dutch city, ruled by a Dutch baron and a Dutch duke for the past x years should be named Dutch, even though the Emperor (I didn't vote for him) may be German.
However, I can imagine a freshly conquered Scottish settlement by the English to be renamed anyway, even though the Scottish baron may still be in power.
I'm thinking about a year thing:
If the [entity] has recently changed hands, it will get the name relevant to the highest ruler. So if the German emperor attacks and gets a set of French baronies, including barons, they will be renamed to their German counterparts.
After, say, 25 years, the naming decision drops a notch. As will it be for each next 25 years until the ruler itself is the name-giver.
Example:
The King of Ireland swears feilty to the (English) Emperor of Britannia. The king has a Scottish duke ruling in North-Ireland, which ended up with a Dutch baron.
Before the feilty, Ireland will have it's Irish name, Northern-Ireland will have it's Scottish name, and the barony will have it's Dutch name (I can imagine Irish baronies not having Dutch names, in which case they should revert to Scottish as part of the Dutchy)
The first 25 years all of Ireland will be English. If the Emperor so pleases, he can remove and replace those nasty Irish and Scottish for some of his own culture, or convert them.
Between 25 and 50 years, assuming the King of Ireland is still in power, the Kingdom will revert to it's Irish name.
Between 50 and 75 years, the Dutchy of North-Ireland will regain it's Scottish name
After 75 years, the Barony will have it's Dutch name, which makes everything as normal again.
Blah. Brainfarting, and this is probably not very true-to-life. Alternatively, just give the lower power the option to choose between the names, and give a relationship penalty if it doesn't choose to adopt the liege's preferred name.