What changed

• Replaced cryptic team_is_exist_err with human-readable error messages
• Updated translations in 12 files across 10 languages

Why

The original error message "team_is_exist_err" was not user-friendly. Now users see clear messages like "Team name already exists" (en-US), "团队名称已存在" (zh-CN), etc.

Test plan

• All language files updated consistently
• No code logic changes, only string values

Closes #1096