For 4.5 (starting w/ Beta 2) and beyond all language strings will be available at the javascript level. To access these strings simply use the following javascript calls:

Code:
SUGAR.language.get('app_strings', 'LBL_LOADING'); // app_strings
SUGAR.language.get('Contacts', 'LBL_LIST_LAST_NAME'); // contacts module
Admins and translators will need to be aware that these javascript language files are cached. If there are any changes to the language files they will need to 'rebuild' the javascript files from the Repair console in the admin section.

This essentially removes the cache files and they will be rebuilt when needed. It also increments js_lang_version in sugar_config so that user's browser will re-cache these js files.

If there are any questions please let me know.