John Mueller of Google answered a question in StackExchange on localization of content. In short, he said you should probably localize all the content on the page, including the alternative text for images, i.e. alt tags.

The question was; “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)?”

The answer given by John Mueller was; “Alt attributes are essentially to tell users what an image is about, so if you have the rest of your content localized, it would make sense to localize those too.”

Alt tags are seen as text on a page by Google, so mixing it up with different languages potentially can cause confusion.

Forum discussion at StackExchange.