Add a missing translation
Adding translations is fairly simple in MODX.
The Core Manager Lexicon
If your language is not in the provided translations, you can fairly simply add a translation of the manager interface by copying the following directory, and renaming to your IANA code for your language (example: "de" for German, "it" for Italian):
core/lexicon/en/ -> core/lexicon/[mylanguage]/
So, for example, if we wanted to create a Haitian translation ("ht"), we would just copy core/lexicon/en/ to core/lexicon/ht/. Then we would edit the .inc.php files in the core/lexicon/ht/ directory, translating the string values there into Haitian.
The Setup Lexicon
A similar process can be done for the installer, if you so choose. Just copy the following directory and rename - similarly to how you do the core manager lexicon:
setup/lang/en/ -> setup/lang/[mylanguage]/
Often, Extras lexicons can be found in their respective directories in the core/components/ directory. For example, Quip's is found here:
You can copy the "en" directory and repeat the normal process to translate it as well.
To contribute to core translations, please join the project on CrowdIn and suggest changes there. With each release, the latest translations are included
More information can be found in this forum post.