Let's be real, the main reason the Spanish vocal-synth fandom doesn't care about Spanish vocal-synths is the same reason the English speaking vocal-synth fandom doesn't care about English vocal-synths; we all got introduced to the concept by Japanese Vocaloids because we're weebs. So obviously, most of us, regardless of where we live on the planet, are only interested in vocal-synths that appeal to what we enjoyed about the Japanese Vocaloids we initially learned about. Not ALL of us are like this obviously, but MOST vocal-synth fans are.
I would actually respectfully disagree with this, for the english speaking side of the fandom at least. I'd say this was true maybe in the early 2010s but nowadays I think a lot of the english voice synth fandom cares a lot about english voice synths actually. I think weeb culture has morphed a lot in the last few years and most people i see nowadays are just "fans of japanese stuff" rather than "zomg japan is awesome and flawless america sucksss! xD owo" and as such i feel like ive seen an increase in fans of engloids and other languages.
In the fandom 10 years ago it was really common for weeb fans to have no idea who any of the engloids were let alone know vocaloids were capable of english, but its really not like that anymore. Theres so so many people i see in the community now that care a lot more about releases relating to their native language or country of origin rather than what's going on in Japan. Obviously not everyone, but a lot more than it seems like you realize.
I think the people only interested in Japanese releases arent so much in the voice synth community but rather more passing fans mostly interested in just Miku.
This is getting a little off topic but I wanted to throw my two cents in here. I've been in this community for a long time, even through my terrible really bad weeaboo phase in middle school, and I've seen this dramatic shift over the years of people actually becoming more interested in the western side of the community.