"I speak German, too."
Translation:Ich spreche auch Deutsch.
It sounds a bit strange, like some writer who tries to sound more than smart ;-). Technically it's not wrong, auch acts as a "Satzadverb" then which modifies the sentence as a whole.
There are some positions where you can put it,
- Auch Deutsch spreche ich (I speak also German, among other languages)
- Auch ich spreche Deutsch (I also speak German, among other people)
- Deutsch spreche auch ich (same as above)
- Ich spreche auch Deutsch (ambiguous)
- Deutsch spreche ich auch (ambiguous)
- (Ich spreche Deutsch auch) (ambiguous and sounds more than stupid, so I don't know if it's grammatical)
- Auch spreche ich Deutsch (ambiguous, both meanings at once)
Since your sentence covers both possibilities of Duo's sentence, it's also ok from the logical point of view. The adverb can act on the noun or pronoun after it or on the verb (and subject) in front of it, that makes some sentences ambiguous. In the first two you can see the logical grouping in bold. They first two words must be a group there, otherwise the verb would not be on the second position any more. In your sentence, there is just the adverb in front of the verb, so it acts on the entire sentence.
PS: One shouldn't forget that the original sentence of Duo is the most natural one that one can use. All others are a bit unnatural, which alters their message slightly.
My take on "Ich spreche Deutsch auch": I recommend not to use it ever, but I might say it in the sense/context of ["Oh, I can't ask you for this German translation, you took Spanish at school."] "No, [I don't only speak Spanish], ich spreche Deutsch auch!", something like "I actually do speak German as well, you know".
Cf. Falco, "Der Kommissar": "Sie fährt ja U-Bahn auch." ;-) (Okay, he's not the best authority.) Or: ["Do you need an extra vegetarian meal?" / "Would you like some grilled zucchini?"] "Ich esse Fleisch auch, if I have to / if you've got any."
"Ich kann Deutsch auch" soll eine akzeptierte Antwort auch sein - ich würde sogar mal sagen, dass eine Sprache "können" umgangssprachlicher als "sprechen" ist, aber zumindest sollen die beide Optionen egal sein.