“to be X” is the passive form, which is does not always or generally comport the same meaning as a true intransitive form. The example you gave is a good one to show it: “to break” in English is actually two different verbs, one is transitive and one is intransitive. They just happen to have the same spelling. The difference is:
Someone breaks the window. (transitive)
The window breaks. (intransitive)
contrast this to:
The window is broken. (passive [with to be])
Since in English, the passive form can only be made from the transitive verb (where the object becomes the subject of the passive sentence), in your case the translation “to be broken” would actually refer to to transitive verb - and that would be wrong.
Again, contrast the two meanings:
The window is broken.
The window breaks.
They are clearly different. But even if you account for the difference in the temporal sense:
(A) The window is in the process of being broken.
(B) The window is in the process of breaking.
When would you say (A) and when would you say (B)?
Would you rather say A or B when you’re talking about a spaceship window being hit by a mini meteorite? It’d prefer B.
Would you rather say A or B when your’re talking about a closed store with valuables in it at night? Here, A would be prefered.
This is because even though it is passive, it strongly hints to someone doing the breaking, even if you don’t know at all who it is; whereas (B) does imply that there is no agent behind it.