Exegesis:

kathōs gar egeneto Iōnas tois Nineuitais sēmeion ‘for just as Jonah became a sign to the Ninevites,’ i.e. by his preaching (cf. Jonah 3.1-5)

houtōs estai kai ho huios tou anthrōpou tē genea tautē, with sēmeion understood, ‘so too will the Son of man be (a sign) to this generation.’ The future tense of estai and of dothēsetai in v. 29 are explained variously (see commentaries). Since the ‘sign of Jonah’ appears to refer to Jesus’ preaching, estai and dothēsetai refer to the immediate future and not to eschatological judgment. For ho huios tou anthrōpou cf. on 5.24.

Translation:

Jonah became a sign, or ‘Jonah’s behaviour (or, what Jonah did) became a sign.’ A similar specification will then be necessary in the next clause.

The men of Nineveh, or, ‘the people/inhabitants (of the city) of N.’; the reference is not restricted to persons of the male sex.

Be to, or, ‘be a sign to/for.’

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.