Well I am not sure how the polylang plugin works, but you mentioned that the post ID stays the same. My plugin only supports a single post meta (subtitle) for each post.
I'd suggest that you use a custom field or create a metabox, to save the other language's subtitle. I have added a filter (called the_subtitle) so you can change the output of get_the_subtitle() but I can't add support for other plugins into mine.... I'd go crazy trying to keep up with everything.