[Phanx]

Now ignores NPCs and enemy players/pets, and won't error on partially-loaded units (eg. players in the process of joining your group). Let me know if you're actually seeing "UNKNOWN" on frames and, if so, under what circumstances.

local CUSTOM_PLAYER_TEXT = "谷歌"
local CUSTOM_PET_TEXT = "PET"

local function SetCustomName(unit, fontString)
	if UnitIsUnit(unit, "player") then
		fontString:SetText(CUSTOM_PLAYER_TEXT)
	elseif UnitIsPlayer(unit) then
		fontString:SetText(strupper(UnitClass(unit) or UNKNOWN))
	else
		fontString:SetText(CUSTOM_PET_TEXT)
	end
end

hooksecurefunc("UnitFrame_Update", function(frame, isParty)
	if frame.name then
		local unit = frame.overrideName or frame.unit
		if UnitPlayerControlled(unit) and UnitIsFriend(unit, "player") then
			SetCustomName(unit, frame.name)
		end
	end
end)

hooksecurefunc("CompactUnitFrame_UpdateName", function(frame)
	SetCustomName(frame.unit, frame.name)
end)

Added/changed parts highlighted in green.