In “whose language is the sun”, I think that the sense of “language” that Spender has in mind is:
language, n. 1.c. An unsystematic or informal means of communicating other than by the use of words, as gesture, facial expression, etc.; non-verbal communication.
Oxford English Dictionary.
That is, children learn through experience, by playing in the sunshine, among the “green leaves” of nature, and this is just as important as learning through the “white leaves” (pages) of books in a classroom.
“Break O break” is repetition for emphasis, the “O” appearing for metrical reasons and “expressing appeal, surprise, lament, etc.” (OED). This rhetorical device is frequently used by Shakespeare, for example:
Charmian O, break! O, break! [Antony and Cleopatra V.2]
Despite the coincidence of wording I doubt Spender meant to allude to this line as the senses of “break” are different. Charmian begs Cleopatra to break (= stop) her suicide, but Spender asks the windows to break (= burst open).
Ghost List, list, O, list! [Hamlet I.5]
Regan Sick, O, sick! [King Lear V.3]
Boy singing Take, O, take those lips away [Measure for Measure IV.1]
and so Spender’s use of it here links with his discussion of Shakespeare in the preceding lines.
In the last stanza of the poem Spender likens the classroom to a catacomb, or tomb, in which the pupils are buried. The images on the wall of the classroom—the picture of Shakespeare, the map of the British Empire, are intended by the teacher to be windows onto the world, but cannot serve that function because the children only have experience of the slum. Spender wishes that the classroom windows would break, to let the children out into the open air, but that’s not enough, so he also wishes that the town (meaning the slum) would break open too, to let the children out into the countryside, to experience “green fields” and “gold sands”.
The image of the schoolroom as a catacomb and the windows breaking open to release the children alludes to the gospel of Matthew:
At that moment the curtain of the temple was torn in two from top to bottom. The earth shook, the rocks split and the tombs broke open. The bodies of many holy people who had died were raised to life. They came out of the tombs after Jesus’ resurrection and went into the holy city and appeared to many people.
Matthew 27:51–53. New International Version.