If you are a Custom Button creator, you have just to :

1. Create a new topic in this subforum with a title of this format

[BUTTON] Button Name

2. Either attach a simple .txt file (please stick to utf-8) on the top post with your strings to translate or simply copy-paste the string list on the top post.

If you are a translator, you can also attach a .txt file with your work to your answer with an explicit name
such as fr-FR_MyButton.txt

An example of topic creation an answer is available here

I have added a page to the wiki on our site that has common words that tend to need translating. This can be consulted by button authors prior to posting here. And they can add the translated terms they need before hand:

Common Strings for Translations

I hope this is a help to our authors, it is in a wiki entry so if you have an account on our site then you can add to it or make corrections.