I respectfully think that teaching only Spanish would be a terrible idea, not because of Spanish or those speaking it.
Teaching Latin makes many languages after Latin trivial: the case system and inflections on the verbs, adjectives, nouns reveal a common theme across many IndoEuropean languages. Russian would too, I believe, having had Russian after 4 years of hardcore Latin. Polish would do better than Russian in this regard, since Russian has simplifications of grammatical distinctions still present elsewhere.
Not faulting any language, as English and Swedish, for examples, simplify things more complex still in German.
I just think if only one language were to be taught, it would be strategic for it to represent more than one present-day set of simplifications of a richer grammatical past, one to which people might be exposed in other modern day languages.
(Spanish and Italian, for examples, collapse/simplify some of the structure present in their main common predecessor. )