Support localization for interface translation

[silkogelman]

Localization for translating the plugin interface doesn't seem to be supported.

"This plugin is not properly prepared for localization (View detailed logs on Slack). If you would like to translate this plugin, please contact the author."
https://translate.wordpress.org/projects/wp-plugins/sparkpost

Not 100% sure if this is a developer issue.

I'd love to translate the interface to Dutch once that's possible.

[rajumsys]

@silkogelman thanks for your interest in localizing the plugin. yeah, i regret to say we intentionally avoided i18n for the plugin for the sake of the speed and wanted to handle it when we see there are interest. however, we would be glad to consider that for future releases.

we'll happily welcome any PR to make it i18n too.