BuddyPress.org

Ah — you mentioned RC1. Then yes, the BuddyPress plugins would be in “mu-plugins” still.

Write in new language to msgstr within buddypress.pot and save as buddypress-spots.mo

Make sure you are creating the MO file from a PO file, which is in turn a copy of the POT that is bundled with BuddyPress.

Your “bp-languages” directory should look something like this:

buddypress.pot

buddypress-mysite.po

buddypress-mysite.mo

If you have trouble editing the POT or PO in a text editor, you can use special software also — see https://codex.wordpress.org/Translating_WordPress and look for “Translation Tools.” In my opinion those are overkill for this type of thing, though.