Goal: 
Propose one or more translations of user interface text, from English to a non-English language, for Drupal Core or a contributed module or theme. User interface text is text that is built into a module or theme, and is visible to administrative users or site visitors.
Skills required: 
Translating
Background and reference information: 
Overview of localize.drupal.org
Translation guidelines for user interface text
Detailed steps: 

You can watch a video that walks through this, and/or follow these steps:

  1. Set up prerequisites: Log in from Common Prerequisites for Contributors.
  2. Visit Localize.drupal.org, where translation information is stored.
  3. Click your language to go the the home page for that language.
    • Most language groups are open for anyone to join -- click the Join button in the right sidebar. 
    • If there is no Join button, read the page to find out how to join the group and follow those steps. 
    • If your language is not on the list, follow the procedure in the right sidebar of the Localize.drupal.org home page to set up a new language group. 
  4. Return to the language group home page if you are not already there. There are three tabs: Overview, Board, and Translate. 
  5. Read the information on the Overview tab to make sure you are following the guidelines for that language. (For instance, many languages have glossaries where they list commonly-encountered terms so that they can be translated consistently.) 
  6. Check the Board tab to see if there is anything you need to read on the translation team's message board. 
  7. Click the Translate tab to see the list of source texts.
  8. Click the "Reveal more filters" link. Filter to find user interface text items in a project that interests you, and a version (release) that interests you. Be sure to choose a project and a version, so you don't end up translating obsolete text. If you don't have a preference for a particular project, check the message boards and translation team home page to see if there are projects the translation team is working on.) Also filter for status "Untranslated" / "Has no suggestion", to limit the list to items that have not already been translated. (You might also browse through some already-translated text items, to get an idea of how to translate new items.)
  9. Find a user interface text item whose meaning you understand and that you know how to translate in the list. On the right side of the page, click the "New translation" box, and enter your translation. If you encounter a term that you are not sure how to translate, check the glossary (if there is one) for your language, or open the translations filtering page again in another browser tab/window and search for another item containing that term that has already been translated, to use as a reference.
  10. Click "Save changes" to add your translation. (As a new member of a language group, you will only be able to suggest translations, and more experienced members of the group will review them before they are distributed.)