please implement WP_LANG_DIR

currently payiban plugin only loads language file from own plugin dir. This makes it impossible to have user specific language files that are loaded from wp-content/languages folder.

Correct way is described here: http://geertdedeckere.be/article/loading-wordpress-language-files-the-right-way

very small change with happy users (well at least me 🙂 as outcome.

https://ww.wp.xz.cn/plugins/payiban-sepa-direct-debit-for-subscriptions/