Many advertisers are quite specific about the audience that they want to reach. One request they often make is that the content reside on English-speaking channels. This means not only that the Influencer Plus video would have to be in English, but that the majority of your content should be in English as well.
This rule is in place to ensure advertisers that their message is reaching audiences who will be interested in their product. If you're unsure of your channel's eligibility for a specific Influencer Plus campaign, feel free to submit a request.