In T272830, @Nikerabbit wrote:

While Wikimedia sites have weekly deployment cycle, third party wikis do not get updated translations unless the upgrade to a new major release. They can run LocalisationUpdate, but in practice nobody does because it needs manual setup and is very slow (if used against github) or takes a lot of disk space if using with local clones of everything).

As an administrator of a third party wiki, I've also felt the update is slow if used against Github as the update.php queries and fetches the translations of all languages from the repository. However, small wikis are commonly monolingual, there are few needs to provide updated translations of all displayable languages that are not the content language of the wiki, omitting other languages is a quite reasonable choice to small wiki administrators. Based on this, I propose to introduce a new option for scoping languages.

# The next line updates all languages
php update.php
# The next lines update only specified languages
php update.php --lang es
php update.php --lang es --lang fi

# The next line updates only the content language
php update.php
# The next line updates all languages
php update.php --all-lang
# The next line updates the content language of the wiki and the specified language
php update.php --lang de

Though I'm not sure which behavior is better, both meet the requirement.