Is there a way to merge multiple decks into one without losing the cards progress ? I have a lot of decks for the same language that I learn and wish I could merge them together.

I think there are a couple options for this.

1. Create a new master deck and nest the decks that you want to merge under this deck. Make sure that Deck settings → Decks → Show cards from subdecks is turned ON in the master deck.
2. Go to one of your decks, use ctrl/cmd+A to select all cards in the deck, on the bulk actions toolbar at the bottom of the page click "Move to" and select the deck you want to move them to. Repeat for each deck you want to merge.