To backup the Custom Dictionary of the Spell Check in ALM 12, you can look for the following path:
C:\Users\%username%\AppData\LocalLow\Mercury Interactive\Quality Center
Look for the file that coicide with the Custom dictionary in use, as shown in the screenshot below:
Keep a copy of the selected file.
In case you want to restore it, place the file in the corresponding path and make sure it be selected as the default custom dictionary.