Try changing your password so they don't have access to your account anymore. It is recommended to change your password once in a while to avoid these kind of situations.
Better take this to Facebook. If your page is hacked, they are the only ones who can help you. But How can you prove it is your page? Think of that before you contact them.
What I would do is if it's in another language and you don't understand it ask a friend to go to their Facebook and look over Theirs to see where the language settings are at to try to guide you to the right page to change your own settings I've done that before it's frustrating
We use Facebook daily to do conversation with our friends, relatives and sometimes with our profession clients too.
It's very important for us to get those conversations back if mistakenly they can deleted.
Some common situations that lost Facebook messages you can recover from:
1. accidentally deleted it on your android phone
2. lost messages after Android 5.0 updated or other Android upgrade.
3. save messages from water damaged, crashed, broken or smashed Android phone
4. Android deleted messages randomly by itself
5. Factory restore Android phone.
And more...
Watch this video and share it with your friends too so that they can also get their precious lost data back.