is it possible to define in the YouTube search the language (or even the country?)?
Yes, they have both country and language support. I've added it to latest connector in latest build (link below). However, the results are kind of weird, more like preference rather than filter. Also seems to work best if you sort by Relevance.
Does not work for me. If I search for "smartphones" with Spanish country and language I get everything except Spanish content...
Try sorting with relevance. Like I said, the results are weird/unreliable.