I think the only way to check the language is:

• In the address bar, type "about:config" (without the quotes), and press enter.
• On the page that loads, accept the warning (you won't be changing anything).
• Search for "general.useragent.locale" (again no quotes).

Look at the value column for that, it should be "en-GB".

Once you're finished, just close the tab. If it is not the right version, you can download and install a different language from https://www.mozilla.org/en-GB/firefox/all/

I believe soon there will be an easier way to identify this (via Help -> Troubleshooting Information, but that isn't released yet).

Thanks.

This works a treat: general.useragent.locale was en-GB.

Useful tip.