You can use the GetString method of the GetString class, passing the key of the string to the first parameter, and the necessary culture to the second parameter:
var ruCulture = new CultureInfo("ru-RU"); var ruString = Properties.strings.ResourceManager.GetString("someString", ruCulture); // strings.ru-RU.resx
If you call GetString with one argument, then the string will be taken from the resource of the culture specified by the current in Thread.CurrentThread.CurrentUICulture
var someString = Properties.strings.ResourceManager.GetString("someString"); // strings.resx
If the string is not found for the culture you specify, then it will be taken from the default resource (resx without language extension)