WebTranslator3 is a web interface for translating OpenTTD. It was released in mid 2009, superseding the old WebTranslator2 interface.
The website can be found at http://translator.openttd.org/
You need to log in to view the translatable strings. If you do not have an account yet, you can easily register. If you already have an account for viewing and want to help translating, send an email to email@example.com in which you state your user name and the language you would like to translate.
Current status here: http://translator.openttd.org/en/status
If you have more questions, please refer to the FAQ.
There is a list of strings that needs validation in WebTranslator3. Strings end up there either if the original English text has changed or if someone has marked the string for validation using the "mark" button. After you have checked that a string is correct or updated it, you press the save button (even if you didn't change anything). Now the string should have disappeared from the list of strings that needs validation.