> it’s not normal in any language to mix alphabets

Decidedly not true. Languages like French and Turkish and German use their own alphabet but they look like the typical “American” Latin alphabet but aren’t. How do you decide if it’s English with non-American letters thrown in or not? Languages like Arabic are transliterated with English letters and Arabic numerals. Other Transliteration dialects use random Arabic characters with Latin others.

By the ‘script’ property, which every Unicode code point has?

It's hardly bulletproof, but it can certainly stop some of the more obvious shenanigans.

That's a good heuristic, but you are still left with spoofing using 100% Cyrillic, and even old school approaches like "app1e.com" (1 instead of l). When you consider distracted users, or people with bad eyesight, you are back to square l.

We (I) really would like system where I would enable the languages/characters that I support/recognise and the rest would be displayed as a number in a box.

> It's not normal in any language to mix alphabets.

This is not true.

For example, my native language has suffix-based articles (there is no separate word "the"; instead there is a suffix attached to the noun). When including a foreign word in the middle of a sentence, some people will attach the article suffix directly to the foreign word, or at best with a hyphen (this is likely not correct "by the book", but some people write like this anyway). With your suggestion, this will show a warning when it is perfectly normal usage.

Furthermore, some Asian languages like Japanese do not use spaces, so when they mix in foreign words or names written in their original script how do you tell that it's intended to be a separate word?