For tho’ it is certainly more laudable, and a thing of greater moment, to be generous, constant, and magnanimous, than merely to be polite and well bred; yet we find, from daily experience, that sweetness of manners, a genteel carriage, and, polite address are frequently of more advantage to those who are so happy as to be possessed of them, than any greatness of soul or brightness of parts are to those who are adorned with those more shining talents.

[E come che l’esser liberale o constante o magnanimo sia per sé sanza alcun fallo più laudabil cosa e maggiore che non è l’essere avenente e costumato, non di meno forse che la dolcezza de’ costumi e la convenevolezza de’ modi e delle maniere e delle parole giovano non meno a’ possessori di esse che la grandezza dell’animo e la sicurezza altresì a’ loro possessori non fanno.]

Giovanni della Casa (1503-1556) Florentine poet, author, diplomat, bishop

Galateo: Or, A Treatise on Politeness and Delicacy of Manners [Il Galateo overo de’ costumi], ch. 1 (1558) [tr. Graves (1774)]
    (Source)

(Source (Italian)). Alternate translations:

And albeit Liberalitie, or magnanimitie, of themselves beare a greater praise, then, to be a well taught or manored man: yet perchaunce, the courteous behaviour and entertainement with good maners and words, helpe no lesse, him that hath them: then the high minde and courage, advaunceth him in whome they be.
[tr. Peterson (1576)]

Although liberality, courage, or generosity are without doubt far greater and more praiseworthy things than charm and manners, none the less, pleasant habits and decorous manners and words are perhaps no less useful to those who have them than a noble spirit and self-assurance are to others.
[tr. Einsenbichler/Bartlett (1986)]