Weather is coming up in French

first make sure language in settings is on english

Second, make sure your Settings > General -> International -> Region Format is set to United Kingdom.

what happens when you click the 'i' symbol?

Try this first:

Reset all settings: Choose General > Reset and tap Reset All Settings.
All your preferences and settings are reset. Information, such as your contacts and calendars, and media, such as your songs and videos, aren’t deleted.

If you decide to restore, you will not have to buy 3.0 again, it's already on your computer.

When you click o the Yahoo! symbol, this launches a Safari page, and takes you to a yahoo site., This yahoo site must be doing some sort of IP address recognition and determining that you are not in the UK.

When I click on that icon in my iPt it takes me to http://uk.m2.yahoo.com/.....

My guess is that the address you are going to has 'fr' in place of the 'uk?

SO you are either in France, or for some reason the Yahoo site thinks your IP address is registered as a French address.

Try going to the yahoo.com and then the weather section... do this directly within Safari on the iPt, and then via a browser on your computer when connected to the same WiFi network that your iPt is connected to).

What happens?

Jenny, can you delete the faulty app and then reload a new version from the App Store. You are lucky to only have one app converted to French. Upgrading to v3 on my ipt converted all text to Greek. Resetting to English improved my Greek a bit!

sounds like it might be time to do a restore, that'll almost certainly fix it.
http://www.apple.com/support/ipodtouch/troubleshooting/ipodtouch/

you've tried restoring using iTunes? only reason i ask is that sometimes people use different terms to describe different processes - and i'm really surprised that it didn't work if you did. did you restore it to new or to the backup? (sometimes the fault can be saved in a backup, so if it doesn't fix it you should try restoring to new)