I agree the multilingual posts tend to be poor quality but I think the original text quality is responsible for the greater part of the difference than the inefficiency of translation. People that think their content is undervalued are more likely to go that route and guess what - their content is mostly valued low correctly. Quality posters without English skills can find audience in their language. Quality posters with English skills tend to post in in English and often feel translating to their first language is not worth it.
I do not want to say multilingual posts should be discouraged but I do not think we are leaking value not treating them efficiently (for lawyers: we are not leaking as much value as the post indicates).