Change language – How to translates theme texts to French?

Hi

I’d like to know how I can change the language displayed on my website.
Some parts of the text are english and as my customers are mostly french, it should be better the full website show only french words.
For example, change “Read more” to “En savoir plus”, or the date presentation in the articles.

Do you know how I can do this?

Thanks for your almighty help!