This issue originally is part of http://issues.ez.no/IssueView.php?Id=17270 . If the main language is non-English, after clearing cache in setup tab, the setup page(only this page) is in english.
I guess the reason is because the translation is not loaded in cache.tpl after clearing cache.
It should better that it redirects to the page(setup/cache) to create a new request, instead of outputs cache.tpl page.
for instance, Portuguese is the main language in admin:
- clear all cache in setup tab (it will be in Portuguese, sometimes English )
- click content structure (it will be Portuguese)
- click setup tab again and clear all cache
- verify the page in setup tab is in English.