I would recommend contacting them the same way as any of your original CSAs and PSAs.
Begin with the welcome email or e-card, introducing yourself as their new co-sponsor. Advise them of your contact information, let them know you are available to help them any time they need it.
Then continue with regular contacts, as you have been doing all along. Regular, informative contacts will let them know you are from their country, and hopefully good rapport will be established.
If their sponsor is not from their country, you may be more of a sponsor than co-sponsor to them, but that will still also benefit both you and your new CSAs. This is the purpose for the Exchange, to get everybody as active as they wish, with leaders they can communicate with successfully.
I would recommend contacting them the same way as any of your original CSAs and PSAs.
Begin with the welcome email or e-card, introducing yourself as their new co-sponsor. Advise them of your contact information, let them know you are available to help them any time they need it.
Then continue with regular contacts, as you have been doing all along. Regular, informative contacts will let them know you are from their country, and hopefully good rapport will be established.