Leaving the default language (Administration > General Configuration > Edit Configuration) set to English, causes the following behavior:

1. As administrator, go to Issue settings, click on Priorities and then Translate
2. Select other language and click View
3. Edit all fields
4. Click update
5. The screen refreshes back to English and loses all data typed.

When changing the default language to the one that is being edit, the behavior do not persist.

This affect customers that use JIRA with English as Default language, but want to present to users in other loactions the possibility of setting language to his own.