Localization of web content is getting more and more attention nowadays. This is due to its growing relevance to SEO and anti-spam practices. So, if you ever wondered if alt tags should be in the same language as the content of the web page – you will now get a concrete answer.
The explanation comes from John Mueller, who is currently working at Google, and he answered a question on localization of content in a StackExchange forum discussion. The concrete question was mixing German and English language:
Do I have to have my static images have alt/title tags in the language of the content or is this not important (i.e. can my German site have English alt tags)?
To this John Mueller answered that the better option is to localize all of the content on the web page, which includes the alternative text for images. According to him if the rest of your content is already localized, it is best if you localize the alt attributes, since in general they tell users what is the image about.
So, if you do not want to cause any confusion to Google or your clients, do not use different languages in the alt tags.