There are huge differences between 'understanding' a different language, as many ex-missionaries do, to actually being able to communicate professionally in that language and then to be able to translate into that different language. The church seems to know this so they have that policy of translating only into ones native language.
If one ignores this one could end up have a 'google translator' standard of translation services!
Yes. you are absolutely right.
However, documents are also translated for a wide variety of purposes.
I agree that the Church generally needs a high level of translation, and I'm not sure that this idea to have members of the community assist will give the Church a high enough level of translation. But, some compromise may give the Church more work done -- some system of training and oversight of volunteer translators may work very well.
Other (non-Church and non-publication) uses may not need as high a level of translation. I often find that Google translations of web pages give me a good enough understanding of a web pages that I can decide if the page is important to me.
I also use google translations as a first draft for many documents that I do translate.
And, perhaps most important of all, we must keep in mind the idea that the perfect is often the enemy of the good
. Or, as the Brazilians say: Quem não tem cão, caça com gato
. (If you don't have a dog, hunt with a cat.)
IMO, too much doesn't get translated at all because we have the idea that a Church-paid professional must do the translation. I think that members who speak languages other than English are often starved for Mormon materials, and are quite happy with a mediocre translation, warts and all, because the alternative is no translation.