"What is it?" she said. "Do zey want us back in ze Hall?"
What is zey, and what is ze? Are they French phoentic alphabets or what?
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Rowling is using alternate spelling to bring across Fleur's accent. In English-speaking popular culture, it's common to stereotype French people speaking English as replacing voiced dental fricatives (the "th" in words like "this" and "the") with voiced alveolar sibilants ("z", as in "zoo")1. So "zey" and "ze" are meant to be understood as "they" and "the", respectively.
"S-s-sorry," sobbed Hagrid, taking out a large, spotted handkerchief and burying his face in it. "But I c-c-can't stand it - Lily an' James dead - an' poor little Harry off ter live with Muggles -"
Harry Potter and the Philosopher's Stone Chapter 1: "The Boy Who Lived"
1 As multiple French-speakers have noted in assorted comments, this stereotype isn't baseless; French people generally do have trouble with the voiced dental fricative, as noted in Graham's answer
The phoneme "th" is not a regular part of the French language. Since French speakers never use that phoneme, they are generally unable to say it. Losing phonemes starts at a pre-verbal level of childhood development, so this is common to everyone who speaks French.
For a similar example in English, many English speakers are unable to pronounce a French rolled "r", because English does not use it; and distinguishing between the two different ways that Finnish uses a rolled "r" (depending on which part of the tongue produces the rolled effect) is even harder.
Of course it is possible to learn the correct pronunciation with a great deal of effort, just as it is possible to learn a new language as an adult with a great deal of effort.
In addition to Jason Baker's excellent answer, I wanted to add that J.K. Rowling worked as a French instructor for several years prior to writing the Harry Potter series; Fleur's accent is, as noted, phonetic to a point, and I would imagine basically authentic, based on J.K. Rowling's knowledge of the French language. I will update this answer if I can find additional confirmation of my assertion.