Your sentence was not added because the following already exists.
Subjects are not proper nouns.
I don't think this is a natural sentence. Could someone who speaks Turkish and English check against that?
This sounds like it could be a natural sentence to me:
A: Is the whole thing going to be in Spanish?
B: The lesson is English. After that, you have to speak Spanish.
Certainly, it might be more common to say "in English", but I think this type of thing does happen in normal speech.
And I'm quite sure "English" should be capitalized.
However, I don't know how well it matches the Turkish.
