First, I'd like to congratulate all teams who worked hard to get to significant completeness of Drupal 8 translations, lead by the Ukrainian and Hungarian teams who are 100% complete and the Spanish team which is close to complete too. The French, Dutch, Danish, Finnish, Japanese and Romanian teams were also hard at work and are all over 70% translated. When you are installing Drupal, there is no special task to do to install in any of the supported languages, just select that on the first screen. The translations ready at that time will be downloaded and used.

Is it a problem at all though if a translation is not complete yet when you install? I don't think so. With prior Drupal core releases, there was a huge significance of complete translations at release time because whatever people used to install Drupal was used to create all their default configuration. Content types, fields, menus, input formats and so on. Even though we have the very handy Localization update module for Drupal 6 and 7, it could not help update configuration that was already created.

In Drupal 8 we solved that problem by applying built-in translation updates to the shipped configuration as well, so if your translation does not have complete coverage yet for the shipped views, fields, contact forms, tours, etc. those will be updated later on seamlessly. Drupal also of course supports making local changes to those which would not be overwritten later with translation updates. To take advantage of this feature (which is not enabled by default), either run a manual update at Administration > Reports > Available translation updates or turn on automated updates with cron at Administration > Configuration > Regional and language > User interface translation and sit back and enjoy your Drupal 8 continually improving with translation fixes automatically.