LANG-90 had quite a few obsolete/useless keys removed. However, a lot of other tasks (that couldn't be completed) showed up as well:

• remove more files with empty keys (samples, categorization, etc.)
• look at pt and pt-br, most likely they're the same
• consider if having a few keys here and there (in Swedish for instance) is of any benefit - it would be probably best to focus on languages suggested by PM

Here are alos suggestions from fgrilli:

Some files like /lang-es/src/main/resources/mgnl-i18n/google-sitemap belong to modules which Magnolia no longer supports and moved to the forge/github.
We could probably move those to their respective modules. However, if someone using that module updates Magnolia to the latest version no longer featuring translations for their language, they will get the English version. Probably not a big deal.
Other files, such as /lang-de/src/main/resources/info/magnolia/ui/admincentral/messages_legacy_de.properties seem to be truly old, 4.5.x related, and probably no longer in use.
However, the status of those modules and files is such a mess that I wouldn't swear on it.