I want to add custom font only for one locale or language of the user, that is, when the user language changes, the custom font will also change.

Hi,

If you are trying to set the  translation for you custom module, you can set it from

Enable debug mode -> settings -> Translation -> export translation 

Then you can choose the language from it, and select your module to add translation

replace the language with the language in which you need to translate and export the file.

Afte exporting a PO file will be generated, Inside that file you can make changes for the fonts that you need to add translation, after add translation you can import the saved files from settings -> translations-> import translation Or you can add the file in your custom module /custom-module/i18n/.po

Hope it helps