Translating interface strings is always a painful process. Actually, it's difficult to achieve in many content management systems, not just in Drupal.
Unlike node contents, interface strings can come from many different places.
They can be part of Drupal core, from modules, themes or dynamically generated by the site's admin.
The hard part is locating these interface strings which appear to site visitors and translating them. To visitors, it doesn't really matter where the strings come from. All user-facing strings must be translated, for the entire page to appear in the right language.
Choosing Strings With The Localization Client Module
The localization client allows searching for untranslated strings easily. For site admins, the most important feature is the ability to spot strings as they are used on public pages.
Once installed and enabled, the Localization Client opens a panel at the bottom of public pages. These pages are viewable to admins only and not to site visitors.
Translation Management adds the Queue for translation button to the Localization Client panel. When you click on it, you're adding the selected string to the translation queue.
Strings that you queue here would be sent to translation from the Translation Dashboard.
Searching for Strings Manually
Drupal's core includes a simple string search tool.
Go to Administer » Site building » Translate interface and click on Search.
Enter the string to search for, choose the string's context (optional) and hit Search.
When you're using Translation Management, you'll notice the same Queue for translation button at the bottom of the search results.
Instead of entering the translations in this page, you can queue string for translation and send them via the Translation dashboard.
Sending the Strings to Translation
Once you've queues strings for translation, head back to the Translation Dashboard.
Go to Administer » Content management » Translation Management » Translation dashboard
You'll see everything that needs translation, including the interface strings that you selected.
This means that you can send interface strings to the same translators who work on the site's contents, without having to teach them about Drupal's string translation facilities.