Can't install language pack

Summary

When we tried to install Japanese language pack ( Confluence-5.9.4-language-pack-ja_JP.jar ) on Confluence, the installation failed with no error.

The installation seemed to be succeeded at a glance but it actually failed.

In spite of the fact that we tried to install Confluence Japanese Language Pack 5.9.4, the dialog indicated that Confluence succeeded in installing 6.0.0-OD-2015.50.0-0611 (the default bundled language pack).

Steps to Reproduce

1. Download Confluence Japanese Language Pack 5.9.4 from http://translations.atlassian.com
2. Install the language pack from Manage add-ons page on Confluece

Expected Results

Language packs should be installed correctly as they were on Conflunece 5.8.x.

Actual Results

Any language pack was not able to be installed.

Workaround

Remove the default bundled language pack and install the new language pack from Manage add-ons page.

Note

Confluence Japanese Language Pack 5.9.4 was released after 6.0.0-OD-2015.50.0-0611 had been released even though the version number is smaller.
It might cause this bug.