In this sentence last week means the week previous to the current week we are in and the week before means the week previous to the week spent in Paris, or two weeks previous to the current week.

If you are asking if you can use last week and the week before to refer to the previous week it is possible. Although the person you are speaking to may not know for sure you are talking about last week if you use week before. Last week refers to the week previous to our current week and is more definite and can be clearly understood. Week before requires additional context for the person to understand what is being said. Without additional information the person you are speaking with may assume 1) you are talking about last week or 2) may ask the question the week before what? Example: I went to the store last week. - This is complete and it is understood I went to the store last week. Example: I went to the store the week before. - Without providing additional context it is unknown when I went to the store. It could have been last week or another time. Hope this helps