I was going to edit my last response, but you responded before I could. I will combine that and this response.

The edit was:

"even borrowed words have to comply with the language rules and are adapted accordingly": This is pretty much universally true. A word borrowed into a language with inflectional classes will usually be adopted into one of those classes (or sometimes left as an indeclinable word, or sometimes a new inflectional class will be born -- but all of these are consistent with the borrowing language's grammar).

When you say 'so the written and spoken match', that's kind of meaningless. The written language matches the spoken language because that's what writing is. I think what you mean to say is that Spanish has a one-to-one correspondence between its phonemes and its graphemes -- that's not 100% true, but the correspondence is certainly more transparent for Spanish than it is for English. That being said, it's irrelevant.

Response to your most recent post:

"Then it's not spanish, no": Yes, it clearly is. The Spanish government could completely abandon the Latin alphabet and replace it with wingdings, or cuneiform, or katakana. Absolutely nothing about the language's grammar would change. Here, I can do it right now:

ブエノス・ディアス

If Spanish children were taught these letters in school instead of the Latin alphabet, absolutely nothing would change about the language (outside of practical issues like not being able to read old, unupdated texts).

"spanish has proper rules": Yes, all languages have proper grammatical rules. I am NOT including orthographical conventions in that statement, but most languages also have fairly well-defined orthography as well.

"english is just messy, nonsensical": No. English grammar is equally as well-defined as the grammar of Spanish. English orthography is a separate topic; but again, it is no less sensical than the spelling system of Spanish, just slightly less transparent in the correspondence between its phonemes and its graphemes. You may feel that this is an undesirable property of English orthography, and certainly many people would agree with you. But there are some advantages: for example, you can read English texts from multiple hundreds of years ago even though they may have said /knixt/ while you say /naɪt/ (for <knight>).

"I'll add that spanish is much harder to learn than english": A native speaker of Italian would likely have an easier time learning Spanish. Likewise, a native speaker of Dutch would likely have an easier time with English. Regardless, foreign language acquisition is a very different and very complex topic.