Plugin localization and page editor error

Hello. I put the Grav in Russian, but the modules (such as Login) in English. And when you edit the page in the editor, when you insert a blank or any other character disappears the written word. How to fix?

You need to ensure all the plugins that have their own language files are translated into your particular language. Also some plugins may not even use any localization, so it really depends. For login plugin for example there is languages.yaml (https://github.com/getgrav/grav-plugin-login/blob/develop/languages.yaml). You can provide a Pull Request with Russian translations for this plugin, or you can just put these into your own file under user/languages/ru.yaml as long as you use the correct nested structure:

PLUGIN_LOGIN:
  ACCESS_DENIED: ...