Bluefish uses aspell for spell checking. If the aspell libraries are not installed on your system, then the spell checking feature will not be available. At the aspell web site you can also download dictionaries for many different languages.
To launch the spell checker, select → or click on the ABC button on the main tool bar. The spell checker will launch in a separate window, which you can keep open as you edit files.
You have the option to check a whole document or just a selection, to use a personal or a session dictionary, and to choose the language depending on the installed dictionaries.
Click on Spell Check to start spell checking the current document.
You may want to set a default dictionary by first choosing the language in the Language pop up menu, then clicking on Set default.
Key words for different languages can be ignored using filters. Currently, the only filter is for HTML. If you want to help write more filters, join the mailing list.