Which kind of stroke causes speech loss?

Several. A stroke that affects broca's area in the dominant hemisphere will affect expressive speech and possibly comprehension. This can be due to ischemic stroke eg a plaque rupture and/or blood clot, hemorrhagic (bleeding) or embolic (drifting blood clot) types of stroke. A stroke of other (posterior) areas may garble the speech but not affect language otherwise.
Left frontal stroke. In general, a stroke of the left frontal lobe can lead to speech loss, or expressive aphasia. There are many different types of speech disturbances which can occur following a stroke. Strokes of the temporal lobe can also lead to disordered speech and difficulty interpreting speech.
Speech center. Damage causes this so any stroke or mass or trauma to this area can affect speech. Look up broca's area.