fyi, i am now finding this in the nginx log for piwik:
2013/03/29 03:34:53 [error] 2478#0: *661076 FastCGI sent in stderr: "PHP message: PHP Fatal error: Cannot redeclare Piwik_LanguageTranslate() (previously declared in /mypath/piwik/plugins/UserLanguage/functions.php:17) in /mypath/piwik/plugins/UserSettings/functions.php on line 285" while reading response header from upstream, client: xxxxx, server: myserver, request: "POST /index.php?module=Referers&action=getSearchEngines&widget=1&viewDataTable=table&idSite=1&period=range&date=2013-02-27%2C2013-03-28 HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "myhost", referrer: "mypath/index.php?module=CoreHome&action=index&idSite=1&period=range&date=previous30"
2013/03/29 03:34:53 [error] 2478#0: *661076 FastCGI sent in stderr: "PHP message: PHP Fatal error: Cannot redeclare Piwik_LanguageTranslate() (previously declared in /mypath/piwik/plugins/UserLanguage/functions.php:17) in /mypath/piwik/plugins/UserSettings/functions.php on line 285" while reading response header from upstream, client: xxxxx, server: myserver, request: "POST /index.php?module=Referers&action=getSearchEngines&widget=1&viewDataTable=table&idSite=1&period=range&date=2013-02-27%2C2013-03-28 HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "myhost", referrer: "mypath/index.php?module=CoreHome&action=index&idSite=1&period=range&date=previous30"