Eowyn says, "Westu Aragorn hal." --- "Be thou Aragorn well."
What Gandalf says to Theoden was actually to Theodred was also Rohirric and is basically the same thing, no sense in giving some sort of proverb in Elvish if the blasted man can't understand it.
He says, "Westu hál. Ferðu, Théodred, Ferðu." which means, "Be thou well. Go thou, Theodred, go thou."
and for good measure --- best put this in languages.