I disagree (and if you want my credentials, I am Mexican). Being able to reason in your own language is a big help. I do this with mathematics in various ways. I can both easily say "sea equis una variable real tal que equis más ye son menor al límite de efe de equis más ye conforme ye tiende al infinito" or whatever in both English and Spanish, and sometimes I prefer saying it in Spanish and sometimes in English. I still count by default in Spanish, although of course I can count in English (or French or Russian). It feels to me like reasoning and language are close in the brain. While you and I have gotten used to thinking of for-loops instead of para-loops, I think there's good reason to think that a learner could benefit from reasoning in their own language.
As far as languages being useless, there's lots of other useless language a person could be learning. That doesn't discredit the raison d'être of these languages.
As far as languages being useless, there's lots of other useless language a person could be learning. That doesn't discredit the raison d'être of these languages.