I have the updated ElvUI 12 and then I updated ElvUI System Datatext to the one available 13 hours ago (3.0.0.) and when I log in I get this error:

16x AceLocale-3.0-6: ElvUI_SystemDatatext: Missing entry for 'System (Improved)'

[string "@ElvUI_SystemDatatext\ElvUI_SystemDatatext-3.0.0.lua"]:549: in main chunk

The 16x in this case seems to vary each time I log in.

Thanks for letting me know. This was fixed in 3.0.1.