Repro steps

Versions used: Confluence 5.4.4 and JIRA 6.2.2, 6.2.3 and 6.2.7.

1. Set up an application link between JIRA and Confluence. In my tests I used TrustedApps as the authentication method.
2. Create a project in JIRA and make sure the field Priority is required.
3. Go to your JIRA user profile and change the language to German.
4. In Confluence, edit a page and insert the JIRA issues macro.
5. In the JIRA issues dialog, select Create New Issue and fill all the fields.
6. Click on Insert.

Problem description

When Confluence tries to create the issue you receive a 400 error from JIRA. The JSON response from JIRA is:

[{"summary":"asdfasdfa sdaf asdf","description":"asdfasdfasdf as","projectId":"10000","issueTypeId":"1","errors":{"priority":"Prioritätsname \"Schwer\" ist ungültig"},"fields":{"priority":"{\"name\":\"Schwer\"}"}}]

The problem seems to be that the priorities get translated and are not picked up correctly by JIRA. This may also affect other default fields with translattions.

Confluence language does not have any effect on it. Issue happens when you change the language in JIRA independently of the language set in Confluence.