Re: My phone has been stuck on Dictionary update for 3 days.
most likely, your keyboard settings (Samsung, Swype, Android,or whatever you have and is being used...) do not have "download over cellular" checked. This essentially means that the download will continue the attempt without success until you activate wifi. This is a bandwith conservation setting, just like in the Google Play Store, where your phone will tell you you have updates available but won't initiate them unless you say so; conversely, if you are already connected to a wifi source, the downloads will begin automatically.
I found the setting for Swype here:
Apps>Settings>(MY DEVICE tab at top of screen)>Language and Input>Swype>My Words>Cellular Data checkbox
I have not found the setting for Android keyboard or the Samsung keyboard yet; however, I would suspect that simply activating wifi and choosing a connection will fix your problem.
Good luck...