If you want to translate some of the words in the website and this word is not available in “Options Panel”, you should follow the translation steps stated above. In addition to what is written above; for example, if your main language is already English and you want to use your website this way, you should use the file that you named fr_FR.po and the sections where you used fr_FR code as en_EN. This way, you can change any word you like in the website by using WordPress Translation System.
Note: While you are changing en_EN.po file by opening it with Poedit software, don’t forget to save after you translate the words.
If you are using WPML, you can do this process with WPML as well.