You'll be upgrading existing English text and translations so that they are translatable with MediaWiki's Translate extension, which greatly improves translation workflow hence making coverage of translated docs vastly broader. You must not add new translations yourself, that's forbidden by the rules. Detailed steps:

1. For a full introduction, read the basic help at https://www.mediawiki.org/wiki/Help:Extension:Translate , the translation tutorial at https://www.mediawiki.org/wiki/Special:MyLanguage/Help:Extension:Translate/Translation_example , the page translation tutorial at https://www.mediawiki.org/wiki/Special:MyLanguage/Help:Extension:Translate/Page_translation_example and the page translation reference manual at https://www.mediawiki.org/wiki/Special:MyLanguage/Help:Extension:Translate/Page_translation_administration .
2. Login or register on mediawiki.org: all your work will happen on that wiki. Pick one page to migrate to Translate from the migration list (on the list's talk page, you can ask any question). Polish the English text as fit/possible to avoid migrating very obsolete/low quality text, skip pages where unsure. Prepare the page for translation (i.e. add <translate> tags etc.) following the page translation tutorial linked above and optionally helping yourself with Special:PagePreparation (see https://www.mediawiki.org/wiki/Help:Extension:Translate/Page_translation_administration#Migrating_to_page_translation ); then edit the migration list, writing next to the page's title that you've done it.
3. A translation administrator (see https://www.mediawiki.org/w/index.php?title=Special:ListUsers&group=translationadmin ) will now approve ("mark") the page for translation at their discretion. When it's approved, complete the job: copy the old translations to corresponding units with Special:PageMigration. You don't need to know the language you're migrating, only to identify which paragraph corresponds to which, even though it's not a loyal translation (machine translation can help grasp the meaning if the layout of the page doesn't provide enough hints).
4. When you're done, add in this melange task a link to the stats for the page (example: https://www.mediawiki.org/w/index.php?title=Special%3AMessageGroupStats&x=D&group=page-Help%3AScreenshots&suppressempty=1 ). Repeat the steps until you've done enough pages. More precisely: to have this task marked done, you need at least one page marked for translation; and a total of at least 120 translation units (i.e. paragraphs/strings) counting those created in the source language (English) and those migrated from existing translations (visible in the stats, row "All languages together", as difference between the columns "Messages" and "Untranslated").
   Hours available: 96
   Mentor: all the communication with mentor(s) about the task will happen on the list page at https://www.mediawiki.org/wiki/Project:Language_policy/Migration_list and its talk page at https://www.mediawiki.org/wiki/Project_talk:Language_policy/Migration_list . Contact the main mentor, Federico Leva (Nemo), via wiki talkpage or on IRC (see https://www.mediawiki.org/wiki/MediaWiki_on_IRC ) directly if you get stuck in any of the steps.

Students are required to read Wikimedia's general instructions at https://www.mediawiki.org/wiki/Google_Code-in_2014#Instructions_for_GCI_students first.