Will you decipher this mysterious inscription?

On the peninsula Plougastel-Daoulas, Brest in Finistere, is a rock engraved with strange inscriptions that no one has ever been able to decipher. The municipality has decided to give a reward to whoever succeeds in translating the mysterious text.

The message is written in a rock next to the shore and is composed of letters aligned, sometimes upside down, group of indecipherable words, three dates 1786, 1789 and 1920.

The inhabitants of the region have no idea who is the person or who is behind this message. They also do not know what the text means.
One big step is to understand which linguage is used. Is it britton, basque ?

Some has a clue ?

Thanks ya’ll!