I think it's a terrific idea and don't think you'd be wasting your time at all! There must be some reason Dora uses it.
My dad spoke fluent French, I sooo wish he'd used it and taught us when we were younger.
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My youngest dd took Spanish 1 & 2 in high school and she was so good. But of course she hasn't used it since and it's been 4 years. It's a gorgeous language and I felt like a lil kid always asking her to please speak Spanish to me.
The local Catholic school teaches Latin, starting in K I think, and while I imagine that might have it's place in Catholicism, I think a child would be so much more apt to use Spanish.
I say go for it!