There are two types of localization in this repository.
For example: addon-title folder includes localization for titles for add-ons.
If the language you want to support is not in the folder yet. Create a folder for your language (for example, create ja.lproj folder for Japanese, and then create a file named Localizable.strings in ja.lproj folder). Look in en.lproj/Localizable.strings for the entries you want to localize, copy the entries to your Localizable.strings and translate.
If the language you want to support is already in the folder. Open Localizable.strings directly in that folder, there should be a comment on each string that tells you the English word this entry. Localize the string and remove the leading // for the translated line.
For example: addon-rich-description folder includes localization for rich descriptions for add-ons.
In the folder find the page you want to translate. The folder names in addon-rich-description are the IDs for add-ons. The folder names in guide-content are the IDs for articles.
https://celestia.mobi/resources/item?item=87D5FBAB-5722-70A9-6D4C-F4FD22EA87BC
https://celestia.mobi/resources/guide?guide=211275CC-D79A-9C81-A2DF-6047DD4AC35D
The above example's add-on ID is 87D5FBAB-5722-70A9-6D4C-F4FD22EA87BC. The article page's ID is 211275CC-D79A-9C81-A2DF-6047DD4AC35D.
In the folder of the page you want to localize. If the page is not translated into your language yet, there will only be an en folder inside it. Copy the folder and rename it to the language you want to localize (for example, ja for Japanese). Open the folder, translate the content in data.html. After you finish translating, open id.txt and change the content to a new random UUID (you can generate one here: https://guidgenerator.com/online-guid-generator.aspx).
If the page is already translated in your language (the language folder exists), and you just want to make some changes, only make changes to data.html.