Alternately, you can heavily annotate a text as you translate it, such that the annotations serve effectively as a language course to teach the reader exactly as much of the source language as is needed to appreciate the context of the translation. This is often done for works in early-modern English, simply because there's enough commonality with modern English that the "language learning guide" can fit in the margins, rather than taking up 90% of the work.

But even with these annotated works, I often wish that the original text was supplied alongside the translation (as it usually is in lyrical translations, e.g. opera), so that the annotations could point to aspects of the original text to explain the choices made, instead of referring to an out-of-band original that's not clear on the page.