No what the commenter was saying was that there are some pretty obvious differences between Spanish and Portuguese (not even remotely comparable to the differences between the mainline variants of English). See also:

https://en.wikipedia.org/wiki/Mutual_intelligibility

That's not what the commenter was saying; he was saying that the different dialects of English are one language because they're all English (one language). This is ultimately circular. I agree with his conclusion, but the distinction between language and dialect is a hazy one and his answer shined no light.

Well, having re-read their sentence... it definitely doesn't read that way to me.

But you can read it that way, if you want to.

No. English is one language because the entire world considers it to be one language. Spanish and Portuguese are two languages because the entire world considers it to be two languages.

That's still circular...

No it's not.

What defines a language? In the absence of context, I can't give you criteria for where the line is between a language or a dialect. But that doesn't matter, because the most important definition for "what's a language vs a dialect" is "how is it perceived by the world?". If the entire world thinks that Spanish and Portuguese are different languages, then they're different languages. It's that simple.

This isn't circular because I'm not saying "they're different languages because they're different languages". I'm saying "they're different languages because people say they're different languages".

It's circular because people say they're different languages because people say they're different languages.