How to translate plugin?
The procedure is standard – you can find the .pot file in the plugin’s languages folder.
Please use PoEdit - https://poedit.net/ to translate the .pot file and create respective .po and .mo fies.
You should upload .po and .mo files created using PoEdit back to plugin's languages folder.
Please name .po and .mo files e.g. bt_plugin-nl_NL.po and bt_plugin-nl_NL.mo (nl_NL is the language code for dutch) and place them in /wp-content/plugins/bt_cost_calculator/languages/.
For other languages code, please refer to the following link: Internationalization.
You can set the site language of your WordPress installation on Settings > General. WordPress will automatically use the right language files if they exist.