Unity-2d needs to build and install mo files even though in Ubuntu it uses lang packs for translations in order to support customized versions.
That is, it is true that in Ubuntu unity-2d translations are delivered by language packs. However, unity-2d should be customizable. If it is customized and there are any modified messages that need to be translated, the Ubuntu lang packs will not deliver translations for these customizations (lang packs don't know about them). In such cases, it is standard practice to use the pkg's _own_ mo files. The pot file in source is updated to reflect the new messages manually or at build time. The po files are updated from the pot file to reflect the current set of messages (manually). Translators complete the translations in the po files. Then, on pkg build, the po files are converted into mo files (run-time per gettext domain, per language translation catalogs). Normally, in Ubuntu, these mo files are stripped from the bindary debs at build time (by pkgbinarymangler). However, in the customization case, they are not stripped. The pkg must install the in the normal pkg location: /usr/share/locale/LC_MESSAGES/<pkg_gettext_domain>.mo. This location takes precedence over the lang pack location: /usr/share/locale-langpack/LC_MESSAGES/<pkg_gettext_domain>.mo. Thus, the pkg delivers customized translations that are used, even when lang packs are installed.
This is the normal mechanism and unity-2d needs to support this or there is no capability to support cusomtizations WITH translations.
To be clear, the unity-2d package needs to be modified to build and install .mo files. This is the standard mechanism most packages perform and we need to do the same.