Writing Basel as Bâle would be silly and non-standard too, yes. Maybe "wrong" is too strong of a classification. But for example, I wouldn't immediately recognize it as Basel in a headline. FWIW Leuven refers to itself in English as Leuven on their own website: https://www.leuven.be/en

Also somewhat interesting to me is that for some reason people think Belgium is mainly a French speaking country, but in fact there are actually more Dutch speakers than French speakers (56% vs 38% native speakers according to Eurostat in 2015) - I have the sense referring to Flemish cities by their French name reflects that