This question may be related to a resolved thread.
I upgraded from 201pb01 to 251 and get the same error as the above when I open the modules page.
Following the advice in that query I disabled the Blogger API module, seeing as I wasn’t using it. That didn’t help. I figured Moblog and Metaweblog modules might be using the same language file so I disabled them too. No luck.
Then I found this problem in the Solspace calendar forum:
http://www.solspace.com/forums/viewthread/5888/#46395
... so taking an educated guess I copied system/expressionengine/language/english/lang.blogger_api.php to blogger_api_lang.php in the same directory, and hey look! It works.
I guess that at some stage the language file names changed but that the change doesn’t get applied when going straight from public beta to 2.5.1. To be clear then, not something I need fixed now, but a tad unsettling. Would like to know that future upgrades are not going to be borked by my hack.
Fritz