Theme’s .pot file is located in /wp-content/themes/avantage/languages folder.
Please use PoEdit – https://poedit.net/ to translate the .pot file and create respective .po and .mo files.
Please name .po and .mo files e.g. nl_NL.po and nl_NL.mo (nl_NL is the language code for dutch). For other languages’ codes, please refer to the following link: Internationalization.
In order to place .po and .mo files properly and not have them deleted when you update the theme, upload them to \wp-content\languages\themes folder. If you don’t have languages folder in your wp-content directory, please proceed and create one. Then create themes folder, and paste the files in it.
Also make sure to translate the theme’s plugin, located in wp-content/plugins/avantage/languages folder.
The steps are the same as for the theme, except the files should be named bt_plugin-nl_NL.po and bt_plugin-nl_NL.mo (with appropriate language code).
For plugin’s .po and .mo files, you should place them in \wp-content\languages\plugins folder.
You can set the site language of your WordPress installation on Settings > General. WordPress will automatically use the right language files if they exist.