Rely on WP.org localization tooling

[kasparsd]

Feature Request

Considering that we don't host any of the translated strings in the plugin source code, we should switch to using the translate.wordpress.org service and WP.org integration per https://developer.wordpress.org/plugins/internationalization/how-to-internationalize-your-plugin/#plugins-on-wordpress-org

This requires updating the Requires at least: version from 4.5 to 4.6:

https://github.com/xwp/stream/blob/bfc0530d617007040ce938f9eb20e7efb87c9fde/readme.txt#L4