Drupal is a great system to run foreign language websites on. The core itself is written in English and modules and themes are expected to follow suit. For developers, very simple wrapper functions are available to mark your translatable strings and let Drupal translate them to whatever language needed. These are the famous t(), the less famous format_plural() and a whole family of other functions. See my cheat sheet (PDF) and the drupal.org documentation for more on this.
Then there is "the other side", whatever does not come from code. Drupal works pretty well and very consistent if you want all of those to be in a foreign language (i.e. not English), but not in multiple languages (any of which can be English at that point). Drupal only has direct multilingual support in nodes (+ fields of entitites) and for path aliases. But life with Drupal means you work with all kinds of other objects like blocks, views, rules, content types, etc which are not "language-aware".
Unfortunately for building multilingual Drupal sites, this is the biggest problem that needs to be worked around. The contributed Internationalization module attempts to fill in the gaps, provide language associations and different workflows for translating these language-unaware objects. This works to some degree, but is really not easy without much help from the modules implementing these objects.