Where do you find the part I bolded? The DMG only says listeners can understand the angel, it doesn't say anything about what the angel can understand (and according to the MM, they only speak Supernal).While Immortals who know Supernal (all of them) can speak the language to be understood by everyone and understand everyone else's language...
Personally I like the idea of an angel who can't negotiate at all. They just turn up, tell you what they want, and expect you to get moving immediately.
The net effect of this is that the PCs already can understand people who normally know Supernal and they can already understand the PC. The only thing the PC gets out of it is being able to speak to Immortals without non-Immortals understanding the PC.
Knowing Supernal also allows PCs to read anything written in Supernal.