Developers can mix modules built in different language modes (such as Swift 5 and Swift 6) in their apps without any issues. Migrating an app to Swift 6 does not require immediate migration of dependencies to the same language mode. It is possible to continue using Swift 5 modules in an app even after transitioning to Swift 6 language mode, and vice versa. This flexibility allows developers to manage the migration process at their own pace, making it convenient and seamless.