I'm sure it must pain the French to know that their language is being ignored, and I'm pretty sure that this is not helping the park's relentlessly shaky reputation. But why is French being dispensed? in favor of English?
There are a few incorrect, unsubstantiated assumptions here. I'm not sure where you derive your certainty about the state of each of these things.
1) It doesn't "pain the French": ask suburban and urban French people younger than 40 or so. It's really not an issue.
2) The French language is not being ignored (see, e.g., the video of Avengers Campus above)
3) Exactly what do you mean by a "shaky reputation," and what makes you think that whatever that means has been "relentless?"
- Theme parks are still stigmatized in France quite a bit compared to how Americans and in particular East Asians (Chinese, Japanese, Koreans) view theme parks, but Disneyland Park (Paris) is no more stigmatized among the French than any other European theme park. In fact, it's seen as a premium experience, as far as theme parks go. Walt Disney Studios Park has an inferior reputation, but that's just because it's an inferior park for a major-name European theme park, which any European theme park fan will tell you.
Do you know any French people under 40? Ask them: in the last 15 years or so, the average educated French person has come to understand most of what's being said in English, and it's largely been because of YouTube.
The French have succumbed to
Globish, basically, at about 80% the level it's taken over Scandinavia. The 1990s were 30 years ago; things change.
Mainstream French (i.e., middle-class consumers and higher-income professionals) generally aren't insecure and touchy about the survival and use of their language anymore. That's now largely a Quebec thing.