We used to try to bundle the language pack inside jira-servicedesk-application.obr but it doesn't get picked up when the obr get installed via UPM.

We figured that the language pack artifact like:
jira-service-desk-language-pack-*.jar

have empty MANIFEST file which make it invalid as an osgi bundle.
We would expect the jar file like:

jira-service-desk-language-pack-es_ES-3.1.0-OD-10-001-v2r62-2016-01-06.jar

having MANIFEST like:
Manifest-Version: 1.0

Manifest-Version: 1.0
Bundle-Description: This plugin contains translations for de_DE
Bundle-SymbolicName: tac.jira service desk.languages.de_DE
Bundle-ManifestVersion: 2
Bundle-DocURL: http://translations.atlassian.com
Bundle-Vendor: Atlassian Community
Atlassian-Plugin-Key: tac.jira service desk.languages.de_DE
Tool: Bnd-2.4.1.201501161923
Spring-Context: *;timeout:=60
Bundle-Name: tac.jira service desk.languages.de_DE
Bundle-Version: 3.1.0.OD-10-001-v2r97-2016-01-07
Manifest-ProductVersion: 1.0
Private-Package: LICENSE.txt,atlassian-plugin.xml,chaperone-i18n_de_DE
 .properties,i18n,sd-projects-i18n_de_DE.properties
Created-By: 1.8.0_66 (Oracle Corporation)

For osgi bundle versioning compliant, please also help to change the generated version to look:

3.1.0.OD

instead of

3.1.0-OD

===========================================================================================================================================
When individual language pack gets uploaded via upm, there's some magic of picking them up and generating a valid MANIFEST. Good thing is you can take the current language packs, upload it via upm and browse the bundle to see how it manifest should look like.

If you generate the jar by maven-jira-plugin, may consider to add instruction similar to:

                        <instructions>
                            <Atlassian-Plugin-Key>com.atlassian.servicedesk</Atlassian-Plugin-Key>
                            <Bundle-SymbolicName>com.atlassian.servicedesk</Bundle-SymbolicName> 

Attached:

plugin_8406482895402974565_jira-service-desk-language-pack-es_ES-3.1.0-OD-10-001-v2r62-2016-01-06_1455504502000.jar
plugin_8189946576377778861_jira-service-desk-language-pack-ja_JP-3.1.0-OD-10-001-v2r117-2016-01-06_1455504517000.jar
plugin_7187231438344317803_jira-service-desk-language-pack-fr_FR-3.1.0-OD-10-001-v2r131-2016-01-06_1455504510000.jar
plugin_7152428401686492404_jira-service-desk-language-pack-de_DE-3.1.0-OD-10-001-v2r97-2016-01-07_1455504490000.jar

Are transformed jars (how language pack should look like to be an osgi bundle/plugin).

Here an example of jira-servicedesk-application OBR with language packs with proper MANIFEST files.
https://s3.amazonaws.com/uploads.hipchat.com/10804/1976430/zo5Sdt2LT4zv72J/jira-servicedesk-application-3.2.0-OD-03-001-SNAPSHOT.obr