Improve internationalization handling

I suggest to take a look at https://developer.ww.wp.xz.cn/plugins/internationalization/ to make the plugin fully available for other languages.

For example Easy_Media_Replace_Helper::trans() is a bad idea because you should not pass variables to gettext functions. The main reason why https://translate.ww.wp.xz.cn/projects/wp-plugins/easy-media-replace/dev/af/default/ doesn’t include any of the plugin strings.