Is there a locale/language pack for my country/language? Where is it?

Check out the Language Packs downloads page.

As a second choice, you may check the forums to see if someone else has recently posted a language pack for your language/Locale.

If you can't find a pre-packaged language pack there, you may be able to make your own. Read all about it here: Localization.

In a newer release, my language file contains translations that do not make sense...

Yes, this is normal. These translations are automatic guesses for new strings you haven't translated yet. They marked as fuzzy and they are NOT ACTIVE until you have made the correction.

When saving the file with POedit, I get errors like "'msgstr' is not a valid PHP format string"...

Recent versions of POedit try to be smart about this but they aren't. All strings in b2 are *not* necessarily printf format string. And in these strings, the use of the % character for other means is legal. POedit does not seem to get this. (Actually it's not really POedit but msgfmt; yet POedit has also problems when trying to be smart about format strings).

Workaround: in "[b]File/Preferences/Editor[/b]", uncheck "automatically compile .mo files when saving" (first option). This will disable the parsing and the warnings. And we don't need to .mo file anyway.