I do think that lang. import could be done from any client... but it is only importing data in client 000.
Watch out, menus are cross client data,
So your menus could be ok but some customizing texts could only be translated in client 000.
That is why you MUST perform supplementation in all clients (including 000)
as supplementation does
1) fill untranslated text with the supplementation lang.
2) copy custo translation to client (formerly done with RSREFILL)