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 W3C Internationalization Checker is a free service by the W3C that provides:
The reports take into account both markup and HTTP headers, which can be particularly useful for
troubleshooting problems. The advice given in the checker reports considers how your HMTL pages (ie. files served as
text/html) will behave in a modern browser, rather than just validating against a particular version of the standard. The advice is tailored to suit files served as
application/xhtml+xml for aspects relating to character encoding and language declaration.
In addition to this checker, the W3C offers 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.
R. Ishida is maintaining the checker at the W3C, using a code base developed with the help of Thomas Gambet.