The localization is treated by the internal WordPress logic. What language you have that language you get.
Well, we (http://prahounakole.cz) were using the ICIT Weather widget for a while and when it stopped to work I went ahead and "fix" it.
It's easy, make your own and put them images in the sub-folder with your custom name to the images folder in this plugin directory. Refresh the widget admin screen. You'll find your pack in the "Image set" Weather widget menu. (good source of icons is:http://www.iconarchive.com/ or http://www.deviantart.com/)
Check how the bundled images how they are done, and follow this naming structure: WORLD WEATHER possible weather cases (image names are 395.png and it thumbnail 395-thumb.png)
If you want to use this widget you have to first get the API from (http://www.worldweatheronline.com/register.aspx)..don't wary it's free, just follow the WWO Policy (http://www.worldweatheronline.com/free-weather-feed.aspx?menu=usage).