Language Match: Ads, landing pages and videos must all be in the same language, and the language must be one that a majority of the target audience will understand.
US: Spanish language ads can link to landing pages in Spanish or English on a case by case basis on Yahoo O&O.
In Taiwan and Hong Kong, ad copy in Simplified Chinese is disallowed.
In Quebec, Canada, ad copy in any language other than French is disallowed.
In France, ads that run on French properties must be in French unless they are targeted to non-French IPs.