How to translate headers; using language file,

1) I already have a translation of the whole buddy press application. The website you suggested
https://premium.wpmudev.org/blog/basic-guide-to-creating-a-buddypress-language-file-for-sitewide-label-and-message-editing/
asks me to do what I already had done. Instead of me manually changing the buddypress/bp-languages/buddypress.pot file I already found and uploaded the Spanish version of BP in to this folder:
buddypress/bp-languages/buddypress-es_ES.mo
perhaps what I need to do is change this code?
Next step is to call the buddypress-mysite.mo file from wp-content/plugins/bp-custom.php like so:
define( 'BPLANG', 'mysite' );
if ( file_exists( BP_PLUGIN_DIR . '/bp-languages/buddypress-' . BPLANG . '.mo' ) ) {
load_textdomain( 'buddypress', BP_PLUGIN_DIR . '/bp-languages/buddypress-' . BPLANG . '.mo' );
}
However, I am unable to find the wp-content/plugins/bp-custom.php file in my files?
HELP please

2) please explain what each are for:
1. BuddyPress Spanish Language .mo file
2. Corporate BP theme language .mo file
3. buddypress/bp-languages/buddypress.pot
Don't you think that #1, should take care of my issue? I obtained the file from http://www.bp-es.com/noticias/descargas/ and placed it in
/wp-content/plugins/buddypress/bp-languages/
Why are there three files to change the language? And what parts of the "front end" does each file change?