Learn Portuguese (Portugal): Ele é meu colega, isto é, trabalhamos juntos. - He is my colleague, that is, we work together.

The word colega is a "two-gender" noun, meaning it doesn't change spelling based on gender. Because the subject is a man (Ele), we use the masculine word for "my" (meu). If we were talking about a woman, it would be Ela é minha colega.

I learned we usually put o or a before possessives in Portugal. Why isn't it ele é o meu colega?

You are correct that European Portuguese normally requires the definite article before a possessive (like o meu carro). However, we often drop the article right after the verb ser (to be) when simply stating a fact, profession, or relationship. Saying Ele é o meu colega is also correct, but dropping the o is very natural here.

Literally, isto é means "this is". Why does the English translation say "that is"?

Isto é is a fixed phrase in Portuguese used to clarify, explain, or rephrase a statement you just made. It serves the exact same purpose as the English expression "that is" or "that is to say."

Why does the sentence use the masculine plural juntos (together)?