Not sure there a way to keep this site set to english only.
If you have the language set as English in the bottom left hand corner of the screen under "Languages," you've basically done as much as you can do. It should stay like that as long as you're logged in, but it's only making the base site English, it's not going to translate video titles or anything since those are written by the uploader. I doubt Iwara is going to bother making translations for video titles either since it would either have to be machine translated (which would mess up song titles, names of characters dancing, etc), or they would have to be manually translated by someone and that's too much work for how many videos, images, and so on get uploaded in multiple different languages.
Okay I understand and most comments I get in everyone video arent in english.
There are web browser extensions out there which allow for near-instant translating through some service, most likely Google Translate.
I'd suggest checking what's available for those so that you can read those comments.
Google Chrome has a auto translate feature. It is far from perfect, but it helps.
Thanks and I'm using google chrome.Not sure it an better overall browser to use your site.