[Phanx]

This will probably (I don't have an active subscription and can't actualy test it) do what you want:

local f = CreateFrame("Frame")
f:RegisterEvent("PLAYER_LOGIN")
f:SetScript("OnEvent", function()

	local myLang, _, _, myTimezone = select(5, LibStub("LibRealmInfo"):GetRealmInfo(GetRealmName()))
	myLang = myTimezone == "AEST" and "AU" or myLang:sub(3)

	hooksecurefunc("LFGListSearchEntry_Update", function(self)
		local leaderName = select(13, C_LFGList.GetSearchResultInfo(self.resultID))
		local realmName = leaderName and leaderName:match("%-(.+)")
		if realmName then
			local realmLang, _, _, realmTimezone = select(5, LibStub("LibRealmInfo"):GetRealmInfo(realmName))
			realmLang = realmTimezone == "AEST" and "AU" or realmLang:sub(3)
			if realmLang and realmLang ~= myLang then
				self.ActivityName:SetFormattedText("%s[%s]|r %s", ORANGE_FONT_COLOR_CODE, realmLang, self.ActivityName:GetText())
			end
		end
	end)

end)

Use Vlad's addon creator tool to turn the code into an addon package.

You'll also need to install the LibRealmInfo library as a separate addon to provide the data.

Rather than hiding the info away in a tooltip, it will just add "BR" (for Brazil, etc.) in orange right before the dungeon name (or other zone/activity name) in the main list so you can easily see it while scrolling. It won't add anything for realms in the same region/language as you.