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Translations are made in Jira native screens only, and third party applications are not covered.
However, we have implemented support for the Portfolio for Jira app and plan to add support for several other widely used apps in the future like EazyBI for instance.
If you have a system that uses REST services to communicate with Jira, then you have some service Jira account that makes changes to Jira entities (using which REST API calls are made). Such service account must have a non-translated locale configured in the user profile, because Jira doesn't support working with translated entities in the services.
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