I have upgraded my site from 4.6x to 4.7.3 for about a month with no significant problems except one: The translated strings keep getting lost everyday. This has never happened even once before the upgrade.
Here are some of the details of this issue:
[1] The copy of the locale.module currently running has the $Id: locale.module, v 1.135.2.1 2006/08/08 17:50:28 killes Exp $. No modification has been done to that file.
[2] I have only one languege activated on the site. It's based on earlier version of Chinese translation with newer translations merged into it. English is not activated. No option for user to select language.
[3] The site can be overloaded during busy hours (SQL connection lost or SQL went away).
[4] The translation strings lost most often are the most common translations (such as translation for "Home", "Mon", "Add new comments", "My inbox" etc.) They got lost with great frequence: Everyday (but not always the same set of strings).
[5] This lost is perminent. Clean cache table won't help. Leave it along will not see any of those translation back.
[6] Those losted translation cannot be fixed with po translation file import with the "merge" mode (keep existing translation and add new ones). The import would complete without error and report "0 translation added".
[7] I tried (on a test installation of my site) deleting the language and recreate it with a po file downloaded from drupal.org. This doesn't prevent the translations from disappearing.