I also use Firefox and had the same problem. My WhatsApp web was being displayed in Spanish, despite my browser language was set to English. I finally fixed it by logging out of WhatsApp web and linking the device again from my phone, but first making it sure that my phone's language is set to English.

Understanding the Context

It turned out that the problem was that I used to change my phone's language from time to time. The phone's language setting is followed by the browser web version, according to this help ... I believe Whatsapp Web should have an option but there's neither an option to start a new conversation with non-existent contacts, nor to add a new contact to the smartphones contacts, nor to the inner whatsapp contacts list. Has anyone a solution to contact this people over Whatsapp from my same whatsapp number?

Key Insights

The web interface does not respect the setting where 'enter' sends a message (or not); pressing 'enter' does indeed send the message. I have composed letters in Word (including line breaks) and then pasted the text into the web interface, but this is overkill. So how does one create a line break in WhatsApp web? Yes. Google Chrome does have some problems with WhatsApp Web.

Final Thoughts

Before trying anything else, you should open an Incognito Window in Chrome and try there. If it works there, it is probably due to cache, cookies, and other site data saved for this web site: Try to clear them and retry. However, I my case, the problem seems to be related somehow with the extensions I use on Chrome. Please, go to Chrome's Settings → Extensions, disable the extensions one by one, and try to open the web site. For ...