Learn Spanish (Spain): El gallego es muy parecido al portugués.

In Spanish, you generally need the masculine definite article (el) when talking about a language as a noun subject, like el gallego here. You only drop the article immediately after certain verbs like hablar, aprender, or the prepositions en and de.

Why do we use ser (es) instead of estar with parecido?

We use ser (es) because being similar to Portuguese is a fundamental, defining characteristic of the Galician language. You use ser for permanent traits, whereas you would use estar for temporary states or appearances.

Why is there an al before portugués?

The adjective parecido (similar) is followed by the preposition a to express what something is similar to. Since we also need the article el for the Portuguese language, a and el combine to form the mandatory contraction al.

Could I just use similar instead of parecido here?

Yes, you could say es muy similar al portugués. Both mean "similar to." However, parecido, which comes from the verb parecer (to seem or to look like), is extremely common in everyday conversational Spanish.