Isn’t maybe the problem that you switched Discourse interface into your own langage? In this case, the translation of the whole page doesn’t often work anymore as you have two languages mixed together (and the first one appearing is even already your own). You have to leave the Discourse parameters on the same foreign language as the forum posts, so that everything is in the foreign language on the page. Translation should work from there.
I just checked on a couple of foreign Discourse, and it’s working for me.
Please provide links of the forums you don’t manage to translate if the above doesn’t solve your problem.