When Jacquenetta enters with her letter in Act 4, she asks Holofernes to read it to her. However, Holofernes ignores her and instead quotes an Italian poet (Baptista Spagnuli of Mantua). In reference to Spagnuli, Holofernes quotes an Italian idiom:
HOLOFERNES
[...] I may speak of
thee as the traveler doth of Venice:
Venetia, Venetia,
Chi non ti vede, non ti pretia.
This idiom translates to “Venice, Venice, / he who does not see you will not admire you.” Holofernes means to imply that those who have not read Spagnuli cannot appreciate him, a sentiment in line with his characterization as a pedant.
There is good reason to believe that this idiom was, in fact, a real Italian saying (not something made up by Shakespeare for effect). The expression actually comes from the writings of John Florio, a translator and contemporary of Shakespeare’s. In his works on Italian grammar, Florio reports the idiom in full as, “Venice, he who does not see you will not admire you, but he that sees you pays dearly for it.”
Florio translated a number of popular expressions and sayings from Italian into English. He also accomplished a landmark translation of Montaigne’s essays into English, and authored an extensive Italian-English dictionary. Shakespeare was known to have read Florio’s work closely, and many phrases attributed to Shakespeare were borrowed from Florio’s translations. In fact, the title of this play, Love’s Labor’s Lost, comes from a line in his work, Second Frutes (“It is labor lost to speak of love”).
Shakespeare's choice to incorporate this phrase into the dialogue could be seen as a veiled allusion to Florio, to whom he owes a significant creative debt. But it also lends a sense of authenticity to Holofernes's character. Holofernes’s choice to use this relatively obscure phrase, particularly in a context which does not require any reference to it at all, underscores his snobbishness and ego. Shakespeare read Florio because he had a deep interest in Italy, as his many plays set in the country attest. But Florio’s work was pretty far outside the mainstream for most readers in England at the time, and Holofernes’ insistence on quoting it reflects his obsession with signaling his level of education at all times.