I'm not a native English speaker, some words are remembered as English(UK), most of the words are remembered as English(US)
Sometimes I feel very much annoying "favourite" and "favorite are counted as a wrong spelling for each other. But I need a spell checker for my careless mistakes. I know one way, to add the words each time I saw it into the dictionary, I'm too lazy for that and it only apply for one machine, I'd like to have this for several different machines.
EDIT: I really hope the spell checker accept both UK and US spelling in one dictionary, so I can type "favourite" or "favorite" up to my mood. I appreciate you are telling me the way to do this, especially on Firefox and Google chrome browser, or the system-wide.
In Chrome it's very simple - go to "chrome://settings/languages" and click on "Add"
In Firefox it's easy to select and add single dictionaries by right clicking on a textarea field (text input with multiple lines) by selecting "Check Spelling" and under "Languages" click on "Add Dictionaries". Unfortunately it won't allow you to select more than one language. But you can hack together a new dictionary yourself if you're comfortable with editing files. You can find the dictionaries by running this from your home directory:
Then you want to combine the contents of the .dic and the .aff files into one of the dictionaries (save a backup). The first line of the .dic file is a count of all the words (# lines minus 1), so you need to update that in the new file. Note that you won't want to update the modified dictionary in the future (unless you made a mistake and need to fix it) and if your default dictionary doesn't show up in that list, then just add it as you would any other language so you can get the dictionary files.
For "system-wide" it really depends on what dictionary the application is using. Larger apps like Chrome and Firefox tend to maintain their own dictionaries.