Flirting is a bonus skill that can be bought in the Lingot store for 30 lingots. Even though it is a skill for the tree, the tree can be finished regardless of whether you complete the skill or not. According to the description in the lingot store: "Do you believe in love at first sight? Learn some French pick-up lines."
souvent, plaisez, glace, amoureux, danser, mannequin, aime
sortir, on, numéro, verre, charmant, aimerais, appelez
beauté, petit, chaude, yeux, petite, café, ciel
Duolingo Lesson: www.duolingo.com/skill/fr/Flirting