It's a good question, and although we would really like to have a multi-language feature, it comes with too many technical complications. Allow me to summarize:
# Exactly how would you be adding multiple languages? For EVERY text-input field in the panel page-settings, it would require us to have multiple fields. For example:
... so on ...
Clumsy at best.
# Then of course, we would need to rip X3 config apart, because we can't just access a variable page.title any more, we would need to access something like page["title_"+language]. Page settings, instead of containing a persistent structure, would have to be filled with multiple language-specific, duplicated items.
# Then, you would need a custom languages-configurator to setup what languages you want to use. All the input fields would then need to get duplicated in the panel depending on what languages you set to use.
# You would need some kinda specific urls for each language /en/, /de/, /fr/ etc. This would require us to somehow create a system, depending on mod_rewrite, that creates pages from the SAME page/folder in X3, but reads in different language files.
# It crashes head-on with caching-system logic, which is necessary to make X3 websites run fast on shared servers. If the language is to be dynamic for the same pages, it would need to re-render templates from the PHP/backend, making output slow.
Of course, as a user, you are understandably not concerned with technical issues and challenges of the application. It's a fair request, but the summary above are reasons why this is not priority for now. We may look into it in the future, but likely alongside a new PHP platform.