"Ich spreche mit dem Herzen."

Translation: I speak to the heart.

[deactivated user]

No, it is not. It is simply wrong and not used in German language at all, and if you try Germans will ask you what you mean with that. Correct ways of telling someone that you say something from the bottom of your heart look here please: http://www.linguee.de/deutsch-englisch/uebersetzung/aus+vollem+herzen.html
It is too long to explain, but don't use that DUO sentence is wrong, you cant speak with you heart.
I am a native German speaker. Cheers

[Sk8rMom]

But you can say "Ich spreche von dem Herzen," right?

[eitan27]

Thanks!

[eitan27]

Is this an idiom? Herzen seems to be plural here, though the translation in English is singular

[magicLamp76]

Herz is a noun with weak inflection form.
Read this and try to learn it by heart.

http://en.wikipedia.org/wiki/German_declension

[deactivated user]

But don't want to tell anyone, that "Ich spreche mit dem Herzen." is a proper German sentence, do you?
As it is impossible to speak with the heart, and there is no idiom in German, this sentence has no meaning and is wrong. There are hundreds of ways in German to tell that you are speaking/talking from your heart, - but none "with your heart" :-)

[deactivated user]

I feel better now, thank you 'backtoschool'