https://wet-boew.github.io/wet-boew/demos/datepicker/datepicker-fr.html
Months are presented in english
This is not a WET issue or even an issue.
Polyfill only load in a specific browser. The browser it does load in the language changes correctly.

The issue you are seeing is browsers that load there OWN date picker (Chrome, Firefox, Most Mobile Browsers, etc). These well only change language when the browser language is changed.
@AtomicNumber28 Assuming @thekodester's reply answers your question, can this issue be closed? Thank you!
I got the same issue. it also happens on many goc websites:
https://www.pc.gc.ca/fr/pn-np/nl/grosmorne/activ/calendrier-calendar
https://www.canada.ca/fr/sante-canada/organisation/contactez-nous/calendrier-evenements.html
This is not a issue as I mentioned before. Browser like Chrome for example set the language in settings. You must change the settings to change the language.
@thekodester thanks, I got your idea finally.
I can't stop wondering if it's not still discriminatory to have someone using a browser setup in a foreign language to them and not be able to make up the date format or the month's name. Should we expect a user to know how or be allowed to change the settings in a browser if they're travelling abroad?
Unfortunately the only way around browser is forcing polyfills which was decide against some time ago
Most helpful comment
This is not a WET issue or even an issue.
Polyfill only load in a specific browser. The browser it does load in the language changes correctly.

The issue you are seeing is browsers that load there OWN date picker (Chrome, Firefox, Most Mobile Browsers, etc). These well only change language when the browser language is changed.
I am just a volunteer on this GitHub project.