SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3 - https://docs.sitefinity.com/administration-custom-error-pages

CustomErrorPagePerLanguagePath_web.config.xml

<location path="bg">
    <system.webServer>
      <httpErrors errorMode="Custom">
        <clear />
        <remove statusCode="403" subStatusCode="-1" />
        <remove statusCode="404" subStatusCode="-1" />
        <remove statusCode="500" subStatusCode="-1" />
        
        <error statusCode="403" path="/bg/error-pages/403" responseMode="ExecuteURL" />
        <error statusCode="404" path="/bg/error-pages/404" responseMode="ExecuteURL" />
        <error statusCode="500" path="/bg/error-pages/500" responseMode="ExecuteURL" />
      </httpErrors>
    </system.webServer>
    
    <system.web>
      <customErrors mode="RemoteOnly">
        <error statusCode="403" redirect="~/bg/error-pages/403" />
        <error statusCode="404" redirect="~/bg/error-pages/404" />
        <error statusCode="500" redirect="~/bg/error-pages/500" />
      </customErrors>
    </system.web>
  </location>