JIRA Portfolio 3.6.0 in JIRA 8.2 custom fields name is on its translation key like below:
JIRA 8.2 (Portflio 3.6)
- Install JIRA 8.2 with JIRA Portfolio 3.6
- Navigate to JIRA Administration > Issues > Custom Fields
- Search for the field "Target"
"Target End" and "Target Start" should be shown
The issue is caused by a bug affecting JIRA Core which is the platform where Portfolio is installed.