Solved....

I have installed Events module today and I've noticed missing language key:

_bx_events_page_title_sys_joined

I added a key thrue Polyglot as:

_bx_events_page_title_sys_joined  = Joined Events

but when on page ....'/page/events-joined' the page title still shows as "_bx_events_page_title_sys_joined" 

The missing key above corrected translation in Studio (..../studio/builder_page.php?type=bx_events)

For the page title solved with:

there was another missing key for Joind Events:

_bx_events_page_title_joined

I've translated it as 'Joined Events' and now the page title for Joined Events shows correctly.