The object of localization is to translate an application's menus, dialogue boxes, labels and other settings (font, character encodings, dictionaries, default search engine etc) for a particular locale.
The Basics of Localization
The user interfaces (UIs) of Firefox, Thunderbird, the Mozilla Application Suite and Sunbird/Calendar are constructed using XML User Interface Language (XUL). This is an eXtensible Markup Language (XML) dialect and it allows the form (layout), function, content (language) and appearance (theme) of the UI to be described in separate files.
Such separation allows applications built with XUL to be localized relatively easily.
All the text of the UI is held in DTD and Java-style
.properties files. These text files link a token used in a XUL file to the actual string to be displayed. So, for example, the following XUL fragment is from am-main.xul:
<caption label="&<b class="token">identityTitle.label</b>;"/> <description>&<b class="token">identityDesc.label</b>;</description>
and corresponds to the following fragment from am-main.dtd:
<!ENTITY <b class="token">identityTitle.label</b> "Identity"> <!ENTITY <b class="token">identityDesc.label</b> "Each account has an identity, which is the ↵ ↳ information that other people see when they read your messages.">
where the "
↵ ↳" indicates a single line, broken here for readability.
By altering the DTD file, you change the text displayed within the application.You can find more information on how to on MDN, along with other documentation.