Using JQL, the fields you search for will use English names for all system fields, but translated names for all custom fields. For example when using Japanese and JIRA Agile, issuekey is in English, but sprint is in Japanese (スプリント):
This may cause confusion for users when typing JQL queries. It would be more consistent if they were all in the same language.
- Install a language pack
- Switch to using that language
- Install a plugin such as JIRA Agile, which has translated custom fields
- Try to search for system and custom fields using JQL