Internationalize issue sidebar title labels and empty labels

Description

Internationalize:

• Todo
• 4 Assignees
• No assignee - assign yourself
• Milestone
• None
• Time tracking
• No estimate or time spent
• Due date
• No due date
• Weight
• None
• 5 participants
• Notifications
• Reference

Out of scope

• UI controls (e.g. Edit) and other elements are out of scope.