Architecture has to be carefully thought through before implemented, or it could all come crashing down at an unexpected moment. You may not realise it, but language is a piece of architecture in all websites. Site builders will be used to thinking about how best to model content, usually in terms of content types, fields and vocabularies on Drupal sites. Every piece of text is modelled somehow - and every piece of text is written in some language. As soon as it matters which language that is - so that translations can be associated with each other and shown beside or instead of one another - that content model needs to incorporate language.