Draugwath, 'tis a mistake not grammatical but merely a lexical one. One can actually "talk english" in a sense of talking about english for example. Rarely seen used this way but still grammatically correct, although op definitely meant speak english as in speak certain language.
Not a mistake. It's what was meant: "to speak <language name>" is a broader expression meaning "to pronounce words in a certain language".
We're having a conversation here discussing various things (well, it was intended

) so i used a more suitable word.
... и добавил:Actually, i was in doubt whether it's better to use the "talk in english" expression but chose another one

Just now i've remembered Radiohead's "Karma police" tune containing a phrase "He talks in maths".
So, probably, i'm wrong a bit
