Hi Titanmanus,
Its probably redeemable if your hours are worth more than the cost of a new one. I did the same - upgrade 2.5.0 to 3.5.0 got the same language error and spent hours working out how to fix it. The info is all on the Internet.
Honestly I can't remember all the steps and I am sure there are shortcuts but here are some of the things I did:
1) There are tools that other people have created that do the above task. use this link with instructions included:
http://www.mediafire.com/download/tk6p7bzc4u09bqb/ftp583v525.rar
Alternatively, you may be able to use the Batch Upgrading tool in "tools.exe" available from Hikvision http://www.hikvision.com/europe/download_more.asp?id=1335 . So long as you can FIND the camera and can LOGIN using admin/12345 (or possibly your own password). If so then try upgrade with the digicap.dav in the above .rar . If the camera was supplied from China then its a Chinese version 2.5 which may or may not have English turned on. However, even if it comes up in Chinese, so long as you can login then you can proceed.
2) Once you upgrade the firmware, if you end up with a Chinese version, to change that back to default English, download PuTTY.exe from www.putty.org/ and 7-zip from www.7-zip.org/ and follow most of the instructions here
(Youtube "How to make Hikvision firmware multilanguage") but rather than commenting out the field in the JSlogin script as he suggested, leave that line alone and instead just change both languages in these two ChooseLanguage statements from ch to en
$("#divLanguageChoose").css("height", "auto");
if ("zh" != $.cookie('language')) {
chooseLanguage("en");
}
} else {
$("#divLanguageChoose").find("label[id=zh]").parent().remove();
if ("zh" == $.cookie('language')) {
chooseLanguage("en");
Then you'll only get English as the default and no options!