She usually talks/ speaks German
This sentence implies that even though the person concerned can speak more than 1 language, she usually communicates in German. I think that 'talk' is correct in this context, since we are not talking about her ability to speak German.
Please, tell me what do you think. Which is the correct option in this case?
speaks is the natural choice.
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See the discussion of the usage of these confusing words here:
http://dictionary.cambridge.org/grammar/british-grammar/speak-or-talk