Veni, Redemptor gentium;
Ostende partum virginis;
Miretur omne saeculum.
Talis decet partus Deo.
Here is a translation by that inimitable translator of hymns, John M. Neale:
Come, Thou Redeemer of the earth,
And manifest Thy virgin birth:
Let every age adoring fall;
Such birth befits the God of all.
As with most of Neale's hymn translations, it is astonishingly good, managing to be both reasonably close to the original and yet eminently singable. Neale translated directly from the original text, and he did an immense amount of hymn translation and writing. (His translations of hymns are still very common; his only original work that is still well known is "Good King Wenceslaus.") Both those features make him probably the greatest nineteenth-century of hymns into English, and one of the best ever.
This is Martin Luther's lovely and popular translation of it into German:
Nun komm, der Heiden Heiland,
Der Jungfrauen Kind erkannt!
Dass sich wundre alle Welt,
Gott solch’ Geburt ihm bestellt.
This is William Reynold's translation of Martin Luther's translation:
Savior of the nations, come;
Virgin’s Son, here make Thy home!
Marvel now, O heaven and earth,
That the Lord chose such a birth.
This is Richard Massie's translation of Martin Luther's translation:
Savior of the heathen, known
As the promised virgin’s Son;
Come Thou Wonder of the earth,
God ordained Thee such a birth.
Incidentally, this Luther-line of the old Ambrosian hymn has attached to the hymn yet another eminent name, that of Johann Sebastian Bach, who wrote music for Luther's words. Bach's harmony has been taken up as a tune (known as Nun Komm) by others in several slight variations.