De la página del Gaim:
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How do I change the language for the Highlight Misspelled words option?
Gaim currently only supports spell checking in your locale language. This is because gtkspell 2 does not offer a good way for us to know which dictionaries are available or to switch between them. This functionality has long been promised for gtkspell version 3, which has been delayed somewhat indefinitely. See gtkspell.sf.net.
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La jodimos Carlos!
, ahora la cosa está en encontrar la manera de decirle al Gaim que el idioma local es el castellano, y mantener el Sistema en inglés (porque no me da la gana de ponerlo toooodo en castellano!)