Does the "rafath" forbidden in the Hadith "When any of you is fasting, he should not rafath (utter lewd words) nor raise his voice" include foreplay such as romantic talk and stimulating kissing, or is it restricted to insults and curses? And how can this Hadith be reconciled with the Hadith "Whoever claims the claims of Jahiliyyah, make him bite his father's penis, and do not use euphemisms"?

The saying of the Prophet, peace and blessings be upon him, "so he should not utter obscenities" is general and includes vulgar talk and foreplay. What you described as romantic, lust-inducing talk is not permissible for a fasting person, as it is a means to intercourse. Kissing for a fasting person, if it incites lust, is not permissible; otherwise, it is. The question about kissing implies it incites lust, and this is at least makruh (disliked).

The hadith: "Whoever claims the claims of Jahiliyyah (Ignorance), make him bite his father's penis, and do not use euphemisms" is an authentic and well-established hadith. It is not considered obscenity (rafath) or transgression (fusuq), but rather a punishment for one who commits the sin of claiming the claims of Jahiliyyah. Explicitly mentioning private parts is permissible out of necessity or for a preponderant benefit, like the Prophet's saying to Ma'iz: "Did you have intercourse with her?" and his saying: "Whoever claims the claims of Jahiliyyah, make him bite his father's penis, and do not use euphemisms." This hadith is general and can be used by both fasting and non-fasting individuals, as it involves a severe reprimand for the perpetrator of this sin.