I learnt French and Latin at the same time, and learnt more about grammar from Latin than I did from French. Mind you, French is similar to English in sentence construction, whereas Latin does have different noun forms and obviously more verb forms which help emphasis the grammatical aspects of sentence construction.
I can understand that - but that is why I wrote "any case-based" language, and did not include French (or Spanish or Italian - because these are not case-based in the main) as examples you could learn and gain the same understanding of English plus learn a real living language.
By the way, I edited my earlier post to better explain my points. I rushed through earlier, and it was not clear what I meant entirely.
Latin is a very straightforward language with mostly consistent rules which provides a good grounding for learning other, modern, more complicated languages. Learning how to say Romani ite domum is all very well but it’s a little bit like mathematics - you certainly won’t need much, if any of the actual knowledge, but the way it trains your brain to think is very useful.
To which I would say: "So what?" To me, this is like saying (since this is a railway group) you will understand better how the modern railway works if you first study how they operated with steam traction, semaphore signalling and vacuum brakes."
I'd say this is perfectly true - but it doesn't mean you have to study those subjects to understand how the modern railway works.
I'd argue that - assuming competent teaching were available - you could do all of the above learning German or Russian (or any Slavic language except Macedonian), or Hungarian or even one of the Baltic tongues (which are, as I understand it, case-based).
And if you really are concerned at the same time about delving into the Latin origin of some English vocabulary, learn Romanian.
But nobody - or next to nobody - does that latter because it's not in our culture.
Don't get me wrong - if you want to learn Latin - fine.
But I don't agree with the argument that it helps you learn other languages or understand English better (because you've never learned or fully understood the grammer of English) versus the language learning options that I've given above. And doing one of those means you end up with a real, working language - even if some options are obscure.