You may already be accustomed to carrying out or managing technical audits for search engine optimisation purposes. There are two key aspects to consider when this involves a global or international site. Firstly, if you don't understand the language on the page, it may simply be difficult to figure out what's going on or what the original intentions of the marketer or designer were.
Nonetheless despite the language, you need to cover all the normal issues - template structure, cannonicalisation, crawlability, robot dynamic loops and so on.
But there are also some specific issues presented by international sites which you will need to look at.
Examples of these are:
Not all of these issues apply in every case - but the more languages and the more countries a web site serves, the more likely you are to encounter these issues - many of which will be covered on subsequent pages.
As far as languages are concerned, it's important to remember that keywords, and therefore keyword density, are language specific - so of course it's important that search engines such as Google can actually figure out what language the web site's pages are in. If they can't figure this - they'll have to make assumptions. Getting those assumptions wrong can seriously affect the performance of your project.
On language detection...
Detecting language one of the first things the search engine does after the crawl because so many subsequent indexation processes depend on carrying out this step. Language detection can also be achieved using many clues such as local domains and IP address (which helps to narrow the range of languages) and then through looking at character sets.
Remember that there are essentially three types of language in the world; international and national languages and dialects. An international language is used in many countries - these include English, Spanish, French, Portuguese, German, Dutch, Arabic and Russian. National languages are strongly associated with a particular country - examples here would be Polish, Czech, Italian, Thai, Norwegian, Swedish, Malay and many others. Dialects are sub-national (though often technically fully-fledged languages in their own right) examples here are Catalan and Basque.
The point is that languages give the search engine a strong clue as to the location of the web site with international languages narrowing things the least and national languages or dialects focusing the location the most.
Certain languages have special characters which are unique to them - this is going to help the language detection software hugely - after that the signals that are looked at are frequently occurring character patterns or words (for instance ‘the' in English).
It should - in theory - be straightforward on most sites. You should be able to find the site in the ‘pages in language' tab in the search engines to prove this point. Apart from this, there may be no obvious signs of a problem other than poor rankings.