<…> If the original text is not in English, is it okay?
If the original text is in a different language than that used within the post, the same convention applies: you need to add the link to the source.
In any case, the source should kind of warrant not falling into being accused of plagiarism, but really, everyone should make a minimal effort to move away from a verbatim copy/[translate][spin]/paste, and come up with something more personalized, be it a summary, a criticism, a series of questions, a versed opinion, or whatever.
From my point of view, it’s pretty annoying trying to discern what is the poster’s original content and what is not, and it’s rather much an irritating waste of time.
Note that the objective of adding the reference link is, in general, to make it feasible to discern what is your own text from what is not (quoting external source fragments would greatly improve giving visibility to this). If there is no source present, the assumption is that the wording on the post is entirely yours (the wording, not the idea). Links can also be added to reference further reading and such.