I can confirm the issue BUT if i remember correctly the bug that causes
this was resolved in the 3.4.2 release so wont be seen in future updates

On 1 July 2015 at 17:39, ste notifications@github.com wrote:

Steps to reproduce the issue

Update a Joomla 3.4.1 to 3.4.2 with the Extension Manager > Upload &
Install Joomla Extension
Expected result

A message translated correctly
Actual result

The green message displayed is correctly
Message
Installing file was successful.

but below it there's a language definition not translated:
FILES_JOOMLA_XML_DESCRIPTION

in some case appears also this string
FILES_JOOMLA_ERROR_MANIFEST

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Brian Teeman
Co-founder Joomla! and OpenSourceMatters Inc.
http://brian.teeman.net/

This was the PR in 3.4.2: #6532 That should fix that issue for the next update. I guess you can test this if you try the upload / install the 3.4.2 package again but i'm not 100% sure else you can update the XML to label the zip file as 3.4.3 for testing ;)

As this has been resolved as stated above I am closing this at this time