Not all pages in the app support live changing of the language. Some pages will update within a few seconds and some need to be backed out of and returned to in order to see the language change.
I assume the language is changed infrequently and think this is a tolerable procedure.
The city name being incorrect is due to how weather underground reports the city name. It reports an observation location and a display location. Currently, the app uses the display location however I am changing this to the observation location in the next update as it is more accurate.
Hope this helps.