It is not mandatory to add this template, but is useful in case where other people might nominate it again, for example if the image is offensive or the licensing might be a matter of debate. For images that are unlikely to be nominated for deletion again, it is naturally less important to use the template.
Put this on the description page or talk page (as seems most suitable) of the image or template.
To add your language to the list of languages supported by this template, please copy the code of Template:Kept/en (or any other language version you prefer to translate) and replace the text strings in there (the form below can be used to create a translation, the English version is preloaded in the edit box). Please change the parameter lang from en (or whatever language you are translating) to the language code of your language.