Don't the English write Bâle instead of Basel sometimes? (An officially germaophone city.) It can be that English retains the French writing. Nothing wring with that.
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
Hard to say! Switzerland and Belgium are both minority francophone countries, so it's hard to use the argument of numbers to justify a French spelling. Maybe because French was a diplomatic language. Maybe because of the Normand invasion.
