Click in the bottom right hand corner (on the flag), then select a language.
@dev:
1. Probably we should do something about this problem?
2. Disable spellcheck by default?
3. Provide dictionaries?
4. Don't highlight anything if there is no dictionary?
The functioning in the trunk is better than in 12.11; in 12.11, if there are no dictionaries, spellcheck automatically disables itself. Unfortunately, if it does find dictionaries, but none of them are its default language, it will enable on an invalid dictionary.
In the trunk, it selects the user specified dictionary, if available, otherwise the system local, if available, otherwise the first valid dictionary, otherwise disable.