Learn Farsi: این خارق‌العاده‌ترین رکوردی است که یک انسان بتواند ثبت کند. - This is the most extraordinary record that a human could set.

Because the clause starts with که (that) and describes a noun modified by a superlative adjective, خارق‌العاده‌ترین (the most extraordinary). In Farsi, when you describe the "best", "worst", or "most" of something, the relative clause uses the subjunctive because it expresses a theoretical limit or potential, rather than a standard factual event.

Why does رکورد have an ـی (i) attached to it to make رکوردی?

That is the restrictive ـی (i). When a noun is followed by a restrictive relative clause (a clause starting with که that defines exactly which specific noun we mean), Farsi attaches this ـی to the noun. It acts like a bridge connecting رکورد (record) to the description "that a human could set".

How do you form a superlative from a long loanword like خارق‌العاده?

Exactly the same way you do with native Farsi adjectives. Even though خارق‌العاده (extraordinary) is a long phrase of Arabic origin, Farsi treats it as a single unit. You simply attach the standard superlative suffix ـترین (tarin) to the end to get خارق‌العاده‌ترین (most extraordinary).

Why do both بتواند and ثبت کند take subjunctive forms in this sentence?

Farsi expresses "can do something" using a conjugated form of توانستن (can) followed by the subjunctive of the main verb. Because this entire relative clause must be in the subjunctive mood (due to the superlative adjective), the first verb takes the subjunctive prefix. It then naturally forces the second verb, the compound (to set/register), to take its subjunctive form as well.