Be sure you pass the following requirements as you follow these 5 steps:
The tool is in a Git repository (or other public location) where the source code can be found (Tools translated by translatewiki.net must be open-source).
To ease the transition from English to multilingual it is recommended you start using the Intuition class in your tool before placing your request here. You can create a local development i18n-file and load it in the class as if it was synced from translatewiki.net to the central i18n directory. Read more about this in the Intuition documentation. By preparing your English messages you will save yourself the trouble of a complicated switch. This way, once it's in translatewiki.net, all you have to do is delete your local file and remove the call to $I18N->loadTextdomainFromFile().
Show users of your tool that it is available in multiple languages and that they should visit the Intuition page to configure their settings. The easier way to do that is to call $I18N->dashboardBacklink() somewhere.
Make a request here by creating a thread (be sure to include an URL to a live version of it as hosted on https://tools.wmflabs.org/)
Modify, Delete or Add messages in the source language.
Include the message-key and text-domain of the message and (if a modification is requested) why it should be modified and your proposed change.