The Flemish consider themselves a distinct people. Their language is a mix of Germanic "Dutch" and French. One feature of the language is the use of double vowels, quite often at the start of a word or name. The double vowel, with its own pronunciation is virtually unheard of in Welsh or English. There is still no formal proof of where William came from, but if you want to "sound it out". It sure "sounds out" as Flemish.