Is taking a sentence in English language and reformulate in french, a plagiarism I am writing my PHD thesis in french, sometimes I took some sentences, phrases from english references and try to rewriting them in french so that the meaning is close. 

Is this a plagiarism?
Yes, basically taking someone's language, thoughts, ideas or expressions without correct referencing is plagiarism...

Edit based on comments:

If you reference the material correctly, even though you translated and / or paraphrased it, is fine.