We've recently launched a glossary feature in GlotPress on WordPress.com. Language validators can create/update the glossary by clicking on the 'Glossary' link in the translation set view (where the strings are listed).
Once terms and translations are added to the glossary, they will show up as tooltips when the term is part of an original string in the translation editor.
You can see how this looks here:
This feature is still under development, but we're really hoping it will make translation work easier. Let us know what you think!