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First, I would check your configuration settings in the administration panel and see if your language is still English. If not, reset it to English.
If that does not work, I would try to re-upload the file:
/languages/english/global.php (this is where _USERNAME and _PASSWORD are defined)
If that does not work, re-upload:
include/common.php (that is the file in which the language file is called)
Regards,
Martijn