Note the original publication of the royal decree in the Moniteur Belge does show accents but also writes Etalle. The municipality website also appears to consistently write Etalle (even when the letter É occurs elsewhere):
https://www.ejustice.just.fgov.be/mopdf/1992/09/24_1.pdf#Page21
https://www.etalle.be/
https://www.etalle.be/en-cours/Zone-de-Police-de-Gaume-RGP-2023.pdf

However, upon further search Étalle does appear to be used sometimes (albeit mixed with Etalle):
https://www.digitalwallonia.be/fr/cartographie/etalle/
https://www.etalle.be/documents/bulletins-communaux/2017-4-decembre.pdf

I'm okay with name=Étalle if you think that's better, but I'll add official_name=Etalle then.