The Realtime tab on the frontpage of the Stack Exchange top level domain does not have any filtering options build in.
It has the purpose of piquing the visitors interest by showing them a stream of new questions that are posted across the network. It is not meant to show you stuff you don't want to see, it is meant to show you stuff you didn't dream of. Even the spam that gets posted (which is why that stream is used for a different purpose as well).
What the designers of the site hope you would do is visit the actual Q/A site, found in the sites list and keep an eye on their front-page. That gives you narrow scoped questions and answers on a topic you're interested in, where some sites offer Q/A in languages other then English.
If you still want to watch the realtime tab but not want to see questions from certain sites you would have to rely on userscripts. I know a couple exist but none I could find that have an actual post I could link you to.
Alternatively you could set-up a filter as user Rob pointed out in a comment