Before doing anything else, get on the mailing list for translators and documenters
(either via the web page
or by sending a mail with "subscribe" in the subject and the body to
(kde-i18n-doc-request AT kde.org)).
This mailing list is being archived with the others at lists.kde.org.
(See the corresponding part of the appendix for more details.)
Second: Take a look at the list of languages and coordinators in KDE.
What follows depends on whether your language is listed on that page or not.
If your language is listed, simply write to the coordinator(s) of your language team and ask him, her, or them how to proceed.
If your language is not listed, post a message to the kde-i18n-doc mailing list asking if anybody is already working on your language or would be willing to join your effort.
It might happen that you do not get an answer from your message on kde-i18n-doc mailing list because nobody is working on your preferred language. Do not worry! First as this is an international mailing list and as most people are volunteers you will probably not get an immediate answer anyway. So please wait at least 24 hours. Especially do not re-send the same message within hours. (Sometimes the mailing list distribution is stalling and in extreme cases it can be for hours.) If after a few days (at most a week), you have not got any answer, then it means that there is currently nobody interested to work on your preferred language. In that case, you can start translating. It would be nice to send a message to tell that you are starting, to avoid thinking that you have abandoned because you have not get any co-worker.
When you start a new translation, you can be added to the language team page. Please contact kde-i18n-doc mailing list to do so.