Announcement: I just published polylang-migrate, a small set of Ruby scripts plus instructions that may help you to transfer a WordPress blog from the Polyglot plugin to Polylang. Should also be easily adaptable for other plugins that use tags to differentiate language-specific content, as Polyglot does.
I have to apologize for the bad software quality, since this is just a one-off script I wrote for a blog migration. So, be sure to make complete backups before using it. Hope it can still save somebody some time in a migration.