Does having other language sub forum affect SEO?

[abdobasha2004]

Does having another language sub forum affect SEO?
I have an Arabic Forum
I would like to add an English subforum for my English visitors and users
does this affect SEO? (decrease or harmful?)

[Alfa1]

Yes, this can negatively affect SEO, because vbulletin does not allow you to set a language for sub-forums. Therefore SE will index forums that are in other languages as the main language set in admincp.
So your English forums will have a lang="ar" instead of lang="en" and are therefore set as Arabic forums.

I am not sure if SE would still classify non-Arabic signs as Arabic, but if not, then SE will not know what language your website is in.

You need a hack to be written to resolve this. You can post a paid modification or a free modification request in the appropriate forum.