Is this a complex sentence? In most countries, people call the game of soccer football.
No. It's a simple sentence. It has only one verb -- "call."
Just to be precise, a complex sentence has an independent clause joined by one or more dependent clauses.It always has a subordinator such as because, since, after, although, or when or a relative pronoun such as that, who, or which. Since your sentence does not meet this definition, it is not a complex sentence.
Yes, the sentence "In most countries, people call the game of soccer football" is a complex sentence.
To determine if a sentence is complex, we need to identify if it contains both an independent clause and at least one dependent clause.
In this case, the independent clause is "people call the game of soccer football." It can stand alone as a complete sentence because it contains a subject ("people") and a verb ("call").
The dependent clause is "In most countries." It cannot stand alone as a complete sentence because it is not a complete thought. The dependent clause functions as an adverbial clause, providing additional information about where or when the action in the independent clause takes place.
Together, the independent clause and the dependent clause form a complex sentence.