Symptoms

JIRA is displaying a mixture of different languages.

Cause

There are two known causes for this behaviour:

1. Due to the way certain Java classes handle internationalization, some action words are dependent on the locale set by the JVM (which uses the system default locale). For example, if the default system locale is set to French but JIRA's default language is set to English some words will still appear in French.
2. A Known issue:  JRA-28392 - Getting issue details... STATUS  which has been fixed in JIRA versions above 6.4

Resolution

Please follow the article Change Locale for Jira server and set the required language and country codes relevant for the JIRA instance.