Learn French: Nous voulons un forfait pour l'été, mais lequel ? - We want a package for the summer, but which one?

Quel is an adjective meaning "which" or "what" and must be followed by a noun, as in quel forfait (which package). Lequel is a pronoun meaning "which one", so it stands entirely on its own to avoid repeating the noun.

How do we know to use lequel instead of laquelle or lesquels in this sentence?

The pronoun has to match the gender and number of the noun it replaces. Here, it refers back to un forfait (a package), which is masculine and singular. If we were talking about a feminine noun like une offre (an offer), we would use laquelle.

Why can't I just translate "which one" literally as quel un?

In English, we add the word "one" after "which" so the word isn't left hanging. In French, you can never translate this word-for-word. The pronoun lequel contains both "which" and "one" together in a single word.

What exactly does un forfait mean in this context?

Un forfait refers to a flat-rate "package" or "plan". It is a very common everyday word in French used for things like ski passes (un forfait de ski), cell phone plans (un forfait mobile), or all-inclusive vacation packages.