How do I filter my Yahoo Answers feed to only show questions written in English.?

You cant there is no way to Filter that Unfortunately

There is no filter on Yahoo Answers, sorry. You can customize your feed (the gear symbol at the top right when you're looking at the category list of questions) but it doesn't include a filter. This is not the first spam-bot attack YA has endured, but it's the worst because when the previous ones occurred, there was staff behind the scenes to take care of the posts before they took over the site. Alas, this time the powers-that-be appear to be unaware or uncaring. We've certainly no indication anyone is paying attention or intending to take action, but that doesn't mean it won't happen.

This YA site is for English language posts only, but spam bots posting in (what we assume is) Chinese have flooded the site. There is no way to avoid seeing their posts unless you happen to frequent a category that hasn't been hit or has been cleared of those posts due to reporting or moving the posts to Travel>Asia Pacific>China.

I've been here for years also and have never seen anything like this. I saw on herr earlier it's Mandarin. But whatever it is it needs to go away.

Have you seen all the other posts about this spammer? Its not wrong language in the wrong site, Its nothing you can filter, its a spam bot attacking the site (quite massively) most are nonsense or illegal posts. You and everyone else are probably going to have to wait until yahoo does something about it.