As a point of clarification, one needs to download the latest nightly build in order to see the three tabs (Global, Overrides, Parameters) at the top of a language plugin accessible via Community Builder >> Plugin Management >> Default language (English)…or whatever your appropriate language plugin may be. Having a CB version of 2.0.15 isn’t enough…you must have the latest nightly build. This threw me for a bit. I kept looking at my 2.0.15 version (not seeing Joomla tell me that there was an update) thinking it was all up-to-date, but I still didn’t see the three tabs. So if you have version 2.0.15 but don’t see the three tabs, just download the latest nightly build.
Joomla can't version check with our nightly builds. So you'll have to check version manually for nightly CB builds for the time being. We'll likely be adding to CBs internal version checker to check for nightly builds if a nightly build is already installed.
forge.joomlapolis.com/issues/6302
Language overrides seem to work for the regular CB languages:
I’ve got this working with the US English (en-us) version of the language file for Community Builder. The only trick I found was that one needs to look-up the “key” from the language file itself. The “key”, of course, is the capitalized text that precedes the “=>” in the language file. Once one enters the correct key, then all is good and the text entered via the “Overrides” tab works great.
We'll be adding a way to find language keys same as Joomla allows once we find the best way to implement it. For now you can edit the language files directly or use full language debug mode within CB > Configuration > Integrations.
Having trouble getting overrides to work with CBSubs:
Using the nightly build files from Nov 17, 2016 I’m still having problems getting the overrides to work with CBSubs…maybe I’m just not doing this correctly. It looks to me like CBSubs is always reverting back to using the language file located at <webroot>/components/com_comprofiler/plugin/user/plug_cbpaidsubscriptions/language/default_language/language.php. I uploaded the cbpaidsubscriptions-en-us language plugin via Community Builder >> Plugin Management. Then entered that plugin, and access the “Overrides” tab, just like above, to change some of the text (namely the paypal purchase confirmation text beginning with “Thank you for your payment of”, i.e. key = THANK_YOU_FOR_YOUR_PAYMENT_OF_S_FOR_THE_S_S_e7bd92), but when I entered the override text, it didn’t take. Multiple tries didn’t work. I then tried to directly change the text in the language.php file located at <webroot>/components/com_comprofiler/plugin/language/cbpaidsubscriptions-en-us/, and that didn’t work either…no changes were effective. Finally, after digging around some I then make direct changes to the language.php file located at <webroot>/components/com_comprofiler/plugin/user/plug_cbpaidsubscriptions/language/default_language/, that those changes worked!! So if one uploads a language plugin, it looks like there are two copies of the language file with CBSubs using the default location.
CBSubs language files are legacy. They aren't needed anymore and eventually will just be rolled into normal CB language plugins. You can add overrides for CBSubs directly in your CB language files and they should work fine as overrides are absolute and always used if a key match is made.
Template overrides don’t work with language files?
As a final try I attempted to do a template override locating the language.php file at <webroot>/templates/prostar/html/com_comprofiler/plugin/user/plug_cbpaidsubscriptions/language/default_language/. However, changes I made to this file didn’t seem to take either. So maybe template overrides just don’t work with the language files.
I don't understand what template override has to do with language overrides. Do not modify core language files. Use backend language override in CB language plugins only and you'll have no issues.