[nicbou]

A few more based on my own experience:

* People don't always speak the language of the country they are in

* There are significant regional differences for a given language. As a French Canadian, I couldn't translate a website to French because it would sound wrong to someone in France. For example, we have vastly different sets of English loanwords.

* The order of things can be different. For instance, German addresses put the door number after the street name. This can break your layout or even your UX in subtle ways.

* You must choose formal or informal pronouns (tu/vous, du/Sie) and use them consistently.

* Labels can make no sense if you don't know what the UI is like. Context is important for translators.