Please follow these steps to translate a theme to your language.
Install WordPress in your language OR Change the language in the admin settings screen. Settings > General > Site Language
Download and install poedit.
Look for default.po file in your theme’s “languages” directory, If there is no such directory then look for default.po in theme’s root directory.
Start translating fr_FR.po in poedit software from English to French.
You do not need to translate everything, instead look for the sentences or words that you really need to translate and leave the others as it is.
Save the fr_FR.po file in poedit and it will create a new file named fr_FR.mo with in the same directory as fr_FR.po file in your computer.
Upload fr_FR.po & fr_FR.mo to the directory from where you have downloaded the default.po in start.
That’s it. Your translation should work fine now.
In case you need any further assistance, Simply got to forum related to your theme and create a new topic for your question.
For Child Theme
If child theme that you are using already has “languages” directory then upload the translated po and mo files to child theme’s “languages” directory instead of parent theme. This way you can update parent theme easily as your translation will reside in child theme.