He sometimes speaks Spanish, which I can't understand.
He sometimes speaks Spanish, which language I can't understand.
Which sentence is correct? Or are all of them possible?
How about the following:
He sometimes speaks in Spanish, which I can't understand.
He sometimes speaks in Spanish, which language I can't understand.
The form A is the preferred one:
He sometimes speaks Spanish, which I can't understand.
The part of the sentence that follows the comma is a relative clause, connected to the noun "Spanish". In that clause "I" is the subject "can't understand" is the predicate. The object of that clause is actually "Spanish" represented by the relative pronoun "which". The pronoun migrates to the beginning of the clause and retains the status of the object.
If we split the sentence into two, we get
He sometimes speaks Spanish. I can't understand Spanish.
Now, why does "which language" sound awkward? Simply put, because it would be redundant. The word "Spanish" in the main clause already has the meaning "language". It's does not mean "architecture" or "history", so there is no need to repeat it.
As others have said, the first form is the correct one:
He sometimes speaks Spanish, which I can't understand.
You could make the second one acceptable by changing the word which to the indefinite article a:
He sometimes speaks Spanish,
which language I can't understand.
He sometimes speaks Spanish, a language I can't understand.
I concur with Victor that the in is optional in both cases, with little difference in the overall meaning:
He sometimes speaks in Spanish, a language I can't understand.
However, this is true for the verb speaks. If we swap out the verb speak for another verb, then the in might become mandatory:
He sometimes says words in Spanish, which I can't understand.
He sometimes says words Spanish, which I can't understand.He sometimes orates in Spanish, a language I can't understand.
He sometimes orates Spanish, a language I can't understand.
Moreover, sometimes the preposition could be included or omitted, depending on what you are trying to say; for example:
He sometimes narrates Spanish, a language I don't understand.
He sometimes narrates in Spanish, a language I don't understand.
I'd be more inclined to use the first one if he is narrating from a Spanish text, but more inclined to use the second one if he is translating English on the fly and thus narrating in Spanish.
In your sentence, the first option sounds more natural and is more common.
However there are times when specifying a noun after which can add clarification and thus be natural.
Let's take the following example, using a language other than Spanish.
He sometimes speaks in an assortment of grimaces and grunts and whistles, accompanied by stares, winks and gestures, which language I don't understand.
Or consider this example:
He sometimes speaks Spanish, not the Spanish of Mexico City or the Spanish of Madrird, not even the Spanish of Barcelona, but the Spanish of the aboriginal peoples of Brazil, which language I don't understand.
In fact, it would also be common to use Spanish once again instead of language:
He sometimes speaks Spanish, not the Spanish of Mexico City or the Spanish of Madrird, not even the Spanish of Barcelona, but the Spanish of the aboriginal peoples of Brazil, which Spanish I don't understand.
