Learn Spanish (Latin America): Cuando lleguemos, comeremos un taco callejero donde tú digas. - When we arrive, we will eat a street taco wherever you say.

Because the exact place hasn't been chosen yet! When donde (where) refers to an unknown, unspecific, or future location, you must use the subjunctive. Here, it translates more like "wherever you say." If you used the indicative dices, it would imply you are talking about a specific place the person always tells you to go to.

Why is the subjunctive also used for lleguemos after cuando?

Just like with donde, Spanish uses the subjunctive after time words like cuando (when) if the action is in the future and hasn't happened yet. Since they haven't arrived yet, "When we arrive" becomes Cuando lleguemos.

Does the adjective callejero come from the word calle (street)?

Exactly! Adding suffixes like -ero or -jero to a noun often turns it into an adjective related to that noun. So, calle (street) becomes callejero (street-related, or "from the street").

Could I say vamos a comer instead of comeremos here?

Yes, absolutely! Comeremos is the simple future tense ("we will eat"). You can easily swap it with vamos a comer ("we are going to eat"), which is extremely common in spoken Latin American Spanish. Both sound perfectly natural in this sentence.