Three ideas I may suggest you.
First one : You can setup an external captive portal. External Captive Portal with External Authentication takes the work of authenticating users squarely on the external captive portal. It can use whatever method it wants to authenticate the station including splash-screen authentication, RADIUS authentication, authentication against an Active Directory server or a proprietary database. User admin pages can be of your language.
Refer Integration guide from extranet. There are samples in that guide.
Second, Use the below Google Chrome plugin to replace English words with words from your language of choice.
https://chrome.google.com/webstore/detail/word-replacer-ii/djakfbefalbkkdgnhkkdiihelkjdpbfh?hl=en
3rd, Use screenscraping. Setup an external web server which does screenscraping.
http://code.tutsplus.com/tutorials/html-parsing-and-screen-scraping-with-the-simple-html-dom-library...
