Hi,

Try refer the following tutorial link.

https://www.cybrosys.com/odoo/odoo-books/odoo-15-development/ch8/

Create a .csv file named res.lang.csv under data directory for adding new language to odoo.

 Eg : Adding Icelandic.

"id","name","code","iso_code","direction","grouping","decimal_point","thousands_sep","date_format","time_format","week_start"
"base.lang_isk","Icelandic / íslenska","is","is","Left-to-Right","[]",",",,"%d-%m-%Y","%H:%M:%S","7"

Activate Language from Settings

Export po file for adding translation

Go to Translations >> Export Translation under settings and select the language from the window and download the po file then add translated terms to the po file 

Add po file under i18n directory

Regards