Different aspect on different browsers
Even though you didn't mention it, this is clearly a multilingual site. Which implies that "language detection" is involved (to decide each time in which language a page (or webform) needs to be displayed.

Your problem related to the usage of French or English language might be because of the "language detection" configuration. 

So check this configuration via Configuration -> Languages -> detection and selection: experiment a bit with the correct order that should be used. Often times, this should be changed to something like 'URL, Default, User', and not something like 'Default, URL, User' (which I assume you're currently using, and which might explain the difference in the language used in different browsers).

The browser differences to display the errors (red boxes, etc) might be some browser dependency and/or something in your sub-theme (custom or not?). With the few details provided it is hard to tell what might help to resolve that part of your question. I recommend you move that part to a different (new) question, or enhance your question to provide more details about the sub-theme.