The projects mentioned above provide a (more or less) easy way to use Apache Solr as a powerful search engine for Drupal. Unfortunately, the only language that works well with them, out-of-the-box, is English.
So if you run a non-English website, you need to tweak all the configuration files by hand or you lose some of the advantages that Solr provides compared to Drupal's built-in database-driven search. Doing so requires a deep knowledge of Solr and search technology in general.
The entire process gets even more complicated if you run a multilingual website.