Your premise is false. There are a lot of Spanish speakers who speaks English too.
And you forgot Google Translator. I read Russian forums (tons of useful information of Russian aicrafts) too and I don't speak Russian, but with Google traslator, I 'm able to understand what they are writing.
I think,it's reasonable ask if we want a Spanish manual when the aircraft is Spanish and many of team members are Spaniards (low development cost, maybe it's profitable). LN relesed a Serbian manual for the Fishbed, for example.
Another interesting question is...The size* of the Russian market is enough to justify a Russian manual?
*Potential buyers.