My website is in a persian language. Can writing "guest posts" on english or russian language websites benefit the SEO for my own website?
I think that overall, content is king. And if you can add great content to your website than this is a plus. If the guest post is off topic or poorer content than the rest of your site then this could dilute your niche with Google.
For instance, if your site is about cars, and you publish a post about icecream in a different language, Google could find this somewhat suspect.
However, it's very common for companies to start stretching out into foreign markets. And the way that is done is by slowly beginning to add more and more content in that language to their website. And so I would consider it a wise marketing move to add alternate language content to your website if it's in your niche. This is especially the case if you intend to continue branching out into that language.