The W3C Internationalization Checker is a free service by W3C that provides information about internationalization features of your page, and provides advice on how to improve your use of markup for the multilingual Web.
Getting internationalization features right at the beginning saves a lot of time and trouble if you ever need to use your content in a language-sensitive way in the future. However, the test will also throw up issues that could cause you problems straight away, such as those related to character encodings and to non-normalized class and id names.
The first part of the report for a page lists key internationalization settings related to character encoding, language declarations, text direction and class/id names. This information includes that coming in HTTP headers, which can be particularly useful for troubleshooting problems.
The second part of the report lists errors, warnings and helpful suggestions based on your use (or not) of internationalization-related markup. The report also points you to resources where you can read more about the topic in question.
The checker is currently a pre-release version, so please let us know about bugs and missing features using the feedback form. We already have plans to add a number of new features over the coming months, and to translate the user interface.
The information panel at the top of the page should work for most formats. In terms of issue reports, the checker currently targets HTML5, HTML4, and XHTML 1.0/1.1 (served as text/html or application/xhtml+xml). It will soon produce issue reports for XHTML5 or Polyglot pages.
The hope is to eventually merge features of this checker with other checkers and validators at the W3C, once it reaches an appropriate level of maturity.
In addition to this checker, the W3C is offering a number of other tools to help you check various types of documents (HTML, XHTML, CSS, RDF, P3P, ...), find broken links in your Web pages, and so on. All these tools are listed on the W3C's QA Toolbox.
The W3C also hosts a number of other Open Source software projects.