There is no formal requirement that your plugin use English as its language. Here is an example of a Russian only plugin.
The plugin repository at wordpress.org is global. You cannot restrict distribution to certain countries. And even if you could, the open source license would allow others to do so. If you release your software under a GPL license, which you must agree to to use the repository, you wave certain rights. One of those is the right to control who is using your software and where.