Learn German: Die Sprache, wohin sich der kulturelle Einfluss verbreitet, ist wichtig. - The language where the cultural influence spreads to is important.

wohin means 'where to'. We use it here because the verb sich verbreiten (to spread) implies movement or direction into something (the language). When a relative clause describes the destination of an action, we can use a directional word like wohin instead of a preposition plus a relative pronoun like in die.

Why is there a sich in the relative clause?

It belongs to the verb sich verbreiten (to spread). In German, this verb is reflexive when something spreads on its own, so literally, the cultural influence spreads 'itself'. It often sounds most natural to place the reflexive pronoun sich right after the relative pronoun wohin.

Why is the adjective ending -e in der kulturelle Einfluss instead of -er?

Einfluss is a masculine noun, and here it is the subject of the relative clause, making it nominative. Because the definite article der already clearly shows us the gender and case, the adjective just takes the 'weak' ending -e.

Why is the relative clause placed right in the middle of the sentence?