Belle doesn’t seem to be used as girlfriend.
From the AHD:
belle
n.
A popular, attractive girl or woman, especially the most attractive one of a group: the belle of the ball.
[French, beautiful, belle, from Latin bella, feminine of bellus; see deu-[sup]2[/sup] in Indo-European roots.]
beau
n. pl. beaus or beaux
1. The boyfriend of a woman or girl.
2. A dandy; a fop.
[French, from beau, bel, handsome, from Latin bellus; see deu-[sup]2[/sup] in Indo-European roots.]
And speaking of belles, here’s one from Italian:
bel·la·don·na
n.
1. A poisonous Eurasian perennial herb (Atropa belladonna) having usually solitary, nodding, purplish-brown, bell-shaped flowers and glossy black berries. Also called deadly nightshade.
2. An alkaloidal extract or tincture derived from this plant and used in medicine.
[Italian : bella, feminine of bello, beautiful (from Latin bellus; see deu-[sup]2[/sup] in Indo-European roots) + donna, lady ; see Donna.]
Don·na
n.
Used as a courtesy title before the name of a woman in an Italian-speaking area.
[Italian, from Latin domina ; see Doña.]
And we can’t forget the Spanish:
Do·ña
n.
Used as a courtesy title before the name of a woman in a Spanish-speaking area.
[Spanish, from Latin domina, feminine of dominus, lord ; see don[sup]1[/sup].]
You’ll have to look up Don yourself . . .