You certainly don’t have to set up a localisation file if changing the odd heading in BuddyPress is all you need to do.
That said – The benefits of having a localisation file in place are huge (i can name some if you like..?) and it isn’t difficult to set one up if you follow the tutorial on that page.
Going back to your method of editing your theme’s files – try making the exact same change in file:
plugins/buddypress/bt-themes/bp-default/members/index.php
If that change works then you haven’t set up your child theme properly.
