Facebook has started rolling out Spanish support for its M Suggestions feature that's part of Messenger. The newly supported language was available within Messenger as of yesterday though at the moment it's only been made available for users in the U.S. That said, all U.S. users should see the new option following an update to Messenger. If an update isn't available then it may take a bit for the support for Spanish suggestions to populate.
Suggestions coming from M in the Spanish language should be automatic, and just as suggestions pop up whenever M is able to detect a relevant piece of text in the conversation that warrants a suggestion, if the text in the conversation is in Spanish and M recognizes this then suggestions that surface should show up in Spanish instead of English. There doesn't seem to be a setting for enabling whichever language you want, so presumably you just speak in the language you want suggestions for. There is some control over suggestions though as you still have the ability to disable suggestions altogether if you don't want them at all.
For right now M Suggestions in Spanish will only be for U.S. users, but this should make sense as M Suggestions in general is still only available in the U.S. which means more language support for Suggestions could surface once Facebook brings suggestions to other countries. When or if that happens is unclear at the moment as Facebook hasn't mentioned anything regarding the expansion of the feature. M Suggestions is Facebook's answer to similar features from the likes of Google Now and Google Assistant, which can make suggestions for things to users based on a ton of different factors. M Suggestions is also just one of the more recent features added to Messenger that's made it an even more popular choice for chat. Messenger Bots are another useful feature that's been made available to users and it's built on artificial intelligence technology just like M Suggestions is. Facebook initially started testing M Suggestions in December of 2016, and rolled it out to all U.S. users back in April of 2017.