Learn German: Das zu übersetzende Kapitel ist wirklich lang. - The chapter that is to be translated is really long.

This is the Gerundivum. It is formed by taking the word zu, adding the present participle of the verb (übersetzend), and attaching an adjective ending (-e). It acts as an adjective that expresses a passive obligation, meaning "which must be translated" or "which is to be translated".

Why are zu and übersetzende written as two separate words here?

Because übersetzen (when it means "to translate") is an inseparable verb. With inseparable and simple verbs, the zu remains a separate word. If we used a separable verb like vorlesen (to read aloud), the zu would be tucked inside to form a single word: das vorzulesende Kapitel.

Why does übersetzende end in -e?

Even though it is built from a verb, the entire Gerundivum structure acts exactly like a normal attributive adjective. It comes directly between the definite article Das and the neuter noun Kapitel in the nominative case. Following the rules of weak adjective declension, it takes an -e ending.

How would I say this using a normal relative clause instead of this structure?

You can express the exact same passive obligation using a relative clause with a modal verb: Das Kapitel, das übersetzt werden muss, ist wirklich lang. The Gerundivum version shown here is simply a more compact, slightly more formal alternative.