fixes #3244

This reverts commit ea10ba49.

Improved `yii\helpers\Inflector::slug` to support more cases for Russian, Hebrew and special characters

Fixes #3687: Default `sourceLanguage` and `language` are now `en-US` in order for i18n formatter to work correctly

Fixes #3716: `DynamicModel::validateData()` does not call `validate()` if the `$rules` parameter is empty

Fixes #3715: Fixed the bug that using a custom pager/sorter with `GridView` may generate two different pagers/sorters if the layout configures two pagers/sorters

[ci skip]

Fixed the bug that requesting protected or private action methods would cause 500 error instead of 404

fixes #3578