Google has just silently rolled out Spanish language support for its Assistant on Android TV, spotted via a new localization page for the product category. There are a few irregularities right now, though, such as inconsistent behavior depending on precisely which setting you flip it to, and Google's support page for the Assistant on Android TV hasn't been updated to mention the new support just yet.
In our own testing with the Nvidia Shield, Spanish is up and working with all expected features if you set it to Español (España), but the US-specific Spanish localization setting seems to have some limits in functionality right now. Although it was able to answer questions posed, actions like controlling smart lights didn't work, so YMMV.
It's surprising it took this long for the feature to land, given non-Android smart TVs with Assistant functionality have had Spanish support since the beginning of October. However unusual the omission and delay may have seemed, Spanish support is (at least mostly) here now. In typical Google fashion, I would expect an official announcement sometime in the next few months, long after everyone who cares already knows it's there.