Add-on store: support localisation for installed add-ons (#15137)
Part of #14973
Summary of the issue:
Installed add-ons do not have their display name and description localised, even if localised manifests exist for the installed add-on.
Description of user facing changes
Reinstate localisation for installed add-ons of the display name and description
Description of development approach
Fetch translated strings from the localised manifests
Testing strategy:
Ensure add-on with localisations is correctly displayed in the installed add-ons tab of the store.
Available and updatable add-ons rely on the strings to be translated when fetching from the store.
