The thing is that that's not the most effective way to learn the basics of a second or third language.
Second-language acquisition is a well-studied field, and at least some teaching in the learner's own language (or a language that they know well) is more effective than immersion.
As an example, I can quickly teach you that almost all words that end in 'tion' in English are the same in French (pronunciation differs, of course), and they are all feminine, with the notable exception of 'translation'. And now you have hundreds of French words. Similarly for Spanish, except the endings are spelt 'ción' and there are a few more exceptions. Repeat with the suffix 'able'. It's the power of language transfer.
information -> information -> información
administration -> administration -> administración
education -> éducation -> educación
masturbation -> masturbation -> masturbación
The reason language schools promote teaching solely in the target language is because it's easier for them: they can teach students with many native languages in the same class.