I am sure that knowledge of English is one of the most important skills for almost everybody. I am far less sure that this includes the level of fluency which enables reading articles on philosophy.

The cost-benefit analysis should take into account utilities of the translators, which may include not only promoting LW ideas, but also promoting their own language (by increasing the number of high-quality texts in that language), improving their own translation skills, spending time they would otherwise procrastinate...