Hello,
Here is a short post that describes the way to translate report parameters in BIRT. It’s not complicated and it works like label or other object type translation but I didn’t find it he first time I looked for it. If you don’t master the translation in BIRT and the « .properties » files, I suggest you to read this article first. You could also check the way to change the Locale of the preview mode in BIRT preview mod in this article.