I'd say paintings can have a language; certain paintings have linguistic content. One that springs to mind is Campbell's Soup Cans by Andy Warhol. If ISO 639-2 codes are used to identify languages, there is a code zxx meaning no linguistic content that could be used for paintings with no text, music with no lyrics, etc. There's also mul for multiple languages. This is one of the cool features ISO 639-2 alpha-3 has over the more commonly used ISO 639-2 alpha-2 codes.