I agree with you that it should be an ornament for every European city or village to have more than just one name. My problem is that I think it's very difficult to impose bilingual town signs if the local populace does not cooperate.
In the Spanish part of the Basque Country there are bilingual signs (Basque and Spanish) and the Spanish part is frequently sprayed over with graffiti. I think it depends on HOW it is communicated and implemented.
The Basque wish for more independence/autonomy is regularly ignored by the Madrid government. Big issue of disrespect.
Besides that some towns have four or five different names, like the city of Lwów (Polish), L'viv – Львів (Ukrainian), L'vov – Львов (Russian), Lemberg (German), Lemberik (Yiddish). All these nations had significant minorities in the past of the city. Do you want to see all there different names on the official town sign?
That is why I wrote at least 40% of the population for at least 200 years to prevent cluttering of a town sign.