The validity and completeness of the manuals in English in relation to other languages

Could the information in the English manuals be more up-to-date or more complete?

Probably not. Apple provides localized information subject to review by a large number of internal and external subject matter experts. However, there is no doubt more attention is given to English than any other language. Their support documents tend to be very accurate, in English anyway. Every word is subject to scrutiny. If they should state something wrong or misleading it can lead to serious financial consequences.

If you should come across inconsistencies or errors in their Polish translation, by all means point them out here and someone can provide Apple corrections for their review. I happen to know they will correct localization (language-specific) errors quickly.

By the way, second question. Is there any website that will provide information about what macOS alb iOS features are unavailable in Polish or are only available in English?

If there are I don't know of any.

to the best of my knowledge, the current English-langauge manuals and articles such as this one:

macOS User Guide - Apple Support

can be instantly shown in the Polish version by selecting Poland as your region. You can change the region using the flag or name on the bottom Right corner of web pages in Apple support.

you could also look at an English-language article, copy and paste the text, and use google translate to see it in one of many different languages.

I expect you will conclude they have already been translated, reviewed, and found to be adequate. All the terminology may not be what you would choose, but I think you will find it consistent across many Apple articles.