Translation is fairly straight-forward. Copy english.xml to locale.xml. Change any messages from English to the target language. As an example:

<message name="UnableToOpenBlob">
  unable to open image
</message>

becomes

<message name="UnableToOpenBlob">
  impossible d'ouvrir l'image
</message>

in French. You then need to copy the locale.xml to the same location as english.xml, e.g. /usr/local/ImageMagick-7/_locale_.xml. Make sure your your locale.xml file is referenced in the locale.xml configuration file.

The final step is to contribute locale.xml to ImageMagick so we can include it in a future release of ImageMagick so the community can benefit from the translation.