Language file of some themes is defined to be POT.
Hint: To detect the difference between PO and POT file, you can refer to this source:
http://betterwp.net/wordpress-tips/create-pot-file-using-poedit/Thus, it certain that you will have to make some minor changes to steps of modifying and translating strings

Please follow these steps:
Step 1: Go to themes > theme_name > languages> open POT file> click Create new translation. After that, you will select a translation language in the drop-down list

Step 2: After you finish translating, you will go to File> select Save As. There will be a new window that allow you to enter name of the file. As we mention above,  name of the file usually includes two parts such as en_US. The first one is code of language and the rest one is code of nation. If you don’t know the code for your language, please refer to this link:
https://make.wordpress.org/polyglots/teams/

Step 3: The result is that two files including mo and po will be generated in themes > theme_name > languages

Refer to related links:https://nootheme.com/knowledge-base/poedit-an-efficient-tool-used-to-translate-or-modify-items-of-a-site/
https://nootheme.com/knowledge-base/avoid-losing-edited-language-files-update/