I haven’t been following the RCS story closely. My impression is it’s a standard core on which each provider can tack on nonstandard extensions, and somehow carriers are involved even though it’s internet-based. It sounds like people who won’t adopt third-party internet messaging apps are going to continue to have a bad time.
As a practical point, saying it in English will almost certainly communicate what you need to communicate. Almost everyone who makes international calls will recognize that you’re speaking English even if they don’t understand what you’re saying, which suggests that the Russian or Korean speaking person they’re trying to reach is not at that number.