That's... not how any of this works.
Out of all of the languages in the world, English is the best at accommodating foreign vocabulary and grammar.
I'm gonna need to see a peer-reviewed source for this. Depending on how you quantify it, I could see an argument for English being "better" at adapting and accommodating foreign vocabulary due to the fact that it often doesn't have to make any overt morphological changes, but plenty of phonological accommodation still has to happen, and English's penchant for dipthongs means that many words don't sound anything like they did in their source language once they've been adapted. So, not only is "best" ill-defined, but it also likely doesn't apply to English.
Also, all languages borrow - the fact that English has more loanwords than many other languages is due to historical accident, not any natural inclination of the language itself. Being the language of a global empire and then the lingua franca for international communication gave English the opportunity to borrow more words - any other language under the same circumstances would have done the exact same thing, whether they had English's history of conquest or not.
the result of all of this is that English is a hodgepodge of Celtic, Latin, proto-Germanic, Old Norse, French, and whatever the people of England spoke before the Celts arrived
This is kind of technically true, but very misleading. English is still very much a Germanic language in grammar and core vocabulary - the vast majority of the 200 most often-used words in the English language, which themselves comprise around 60% of the English corpus, are native English words, for example.
Even the most frequent 1000 words [are 83% English in origin][(https://en.wikipedia.org/wiki/Foreign-language_influences_in_English).
90% of the language is loanwords.
This is incorrect - by most guesses, and as far as we can tell, 70%-75% of the English corpus is loanwords.
This is also nowhere near the language with the highest percentage of loanwords. Armenian is estimated to be close to 90%, for example (though, again, native Armenian words make up a large percentage of the core of the corpus - this is the usual pattern when it comes to borrowings).
So I would say that in order to be fluent in English, you have to know at least a little bit of Celtic, Latin, German, Norse, and French.
This isn't how language works. Once a word has been borrowed, it become a part of that new, self-contained linguistic system completely divorced from its original form and usage in the source language going forward. We know this is the case both because it usually must undergo phonological changes in the adaptation process (often to the point where it's unrecognizable to speakers of the source language), but mostly because once it's borrowed, the meaning is free to (and often does) change from what it was in the source as well. Not to mention the fact that given enough time uninformed speakers usually have no idea which words are native and which are borrowed.
A fluent monolingual English speaker would be just as mystified by a person speaking Icelandic as they would by a speaker of Warlpiri, because having a few thousand-year-old Norse borrowings doesn't give English speakers any intuition whatsoever into Icelandic as a cohesive linguistic system.
I think Anglos have no bloody excuse for being unwilling to learn a new language.
The vast majority of them have no reason to, and would see no benefit from doing so. I say this as someone who does speak another modern language, and who reads ~10 ancient ones passably.
there isn’t an Anglo alive without any talent for learning new languages
Once again, technically true, though likely by accident. Current consensus on language acquisition is that, while aptitude for learning new languages does seem to decline over time, all speakers do likely retain the ability to learn new languages throughout their lives. English speakers see no benefit here over speakers of other languages.
It came free with your English!
Again, this is false, because that's not how language works, for the reasons noted above.
Note that this isn't me making a statement about your overall point in this post. I think it's great to learn new languages - it's part of why I became a linguist - I'm just trying to correct some of your misconceptions both about English in particular and about language in general.