Plugin translations

Hi,
i’m trying to translate strings that appear on the user front-end to my language.
I uploaded .po and .mo files with translated strings to the /lang/ folder of the plugin.
I’m trying to translate to Slovenian, so I used sl-SI as file name.
As that didn’t work I tried sl_SI, then wp_braintree_sl_SI and wp_braintree-sl_SI, none works.
What am I doing wrong?

On a related note: some string appearing on the front end are missing from the default.po file. And some appear in the mce_lang.php file as well. What’s going on?

Thanks for helping out!