It seems that there are growing concern on the language problem with the AdSense code. Most likely, people reported that all their ads have been changed to Japanese or Chinese, even if they have chosen English to be the language of the site or advertisements. Even Google has not noticed this problem and has a corresponding post in its blog. First of all, if you use the HTML editor to preview the ads, the ad may change to other language. It is absolutely normal, and you do not need to worry about it. Even if you have uploaded it onto the web, the ad may actually be shown in different language. This is because Google's bot may take up to 48 hours to crawl your site. Before that, irrelevant ads or wrong-language ads may be shown. Again, Google has assured that this is normal.
The paragraphs below is quoted from AdSense blogs: "Before our system can display targeted ads on your pages, our crawler first needs to access your pages." While you're building your pages within your HTML editor, your page has not yet been crawled , So you may sometimes see less-targeted ads or even ads in foreign languages. However, after you've published a page to the web, you should start seeing ads in the appropriate language. Please also keep in mind that it can sometimes take 1 to 2 weeks for our crawler to revisit your site after you've made any changes, but your ad relevance should improve over time. As always, we appreciate your patience as the crawler continues its work. "Google has advanced technology to detect what Your content is about, and deliver targeted ads to your site. This is why Google is so successful in its internet advertising industry.