We are using the following package
https://pub.dev/packages/google_mobile_ads
But our test interstitual ads and rewarded ads are in the wrong language. Instead of english (which is what the device locale is set to) we get a spanish ad. Is the any way to get those ads matching to the users device locale? We are currently testing on IOS
Google AdMob will target Ads by its own, it will use the device and app locale to know what Ads should they serve to this user.
The issue that you are experiencing is most probably caused by advertiser filling Spanish text in English fields when they try to run a campaign, or it is a result of mistakenly targeted ad campaign (AdMob gives advertiser the ability to serve a Spanish ad to English speaking users intentionally)
From Google Help Docs
Our technology can determine the primary language of a page. If your content is in a language supported by our program, AdSense will target ads in the appropriate language to your content. We also target ads to match the language settings specified by advertisers in their advertising campaigns. Language targeting allows advertisers to choose the language of the sites that they'd like their ads to appear on.