[chaoky]

The central axiom of linguistics is that no language is inherently more expressive than another. Yes, that means that conjugation and declension is no more complicated than strict word order. Grammatical gender provides redundancy, and conjugation allows for subject pro-drop. Orthography has nothing to do with the actual spoken language and things like "phonetic" pronunciation are not really an intrinsic feature of any language. Any language can be matched with a phonemic orthography; the only reason written English hasn't been is due to historical inertia.

[cbr]

Let's say we pick a random person and make them learn a completely unrelated language. Sometimes they will learn faster than others, like Spanish or Swahili vs English or Mandarin. Why would we expect them all to be the same difficulty?

[umanwizard]

An easy-to-learn writing system is a massive help when learning a language, but the writing system isn't part of the "language" as linguists understand the term. Just a representation of it.

So I totally believe that it's easier to learn English than Mandarin (and maybe easier to learn Spanish than English) because the writing system gives you more hints, but that doesn't have anything to do with the (spoken) language, strictly speaking.

On another note, it's not clear to me that "as phonetic as possible" makes a (written) language easier to read, since our brains process words/morphemes as chunks, rather than sounding them out letter-by-letter (this is how reading Chinese is even possible). So semantic vs. phonetic writing should be thought of as a tradeoff. As an easy example: spaces between words are not phonetically justifiable (there is no pause between words in natural speech), but they sure help reading comprehension a lot.

[chaoky]

It's not a question of inherent difficulty, which doesn't make sense to quantify absolutely. It's all about how similar it is to the random person's native language, phonologically, morphologically, syntactically etc... Obviously Portuguese would be faster for a Spanish speaker to learn, since they are very similar in many aspects (in fact, they are both very conservative Iberian Romance languages) than it would be for a Spanish speaker to learn Polish. On the other hand, Polish would be easier to learn than Spanish for a Czech speaker. None of these are objectively more "difficult" or "complex" than one another. They are just "different".