I would try (for an 8 year old dog)
1.) a visit to the vet.....if not feesible or dooable -trust me I get it and I have NOooooo idea how spendy a sedation and cleaning is in France and shudder to think what my el Cheapo vet here would charge. because I just had to have one of Casper teeth surgically removed and the bill was $30.00 but I'm on the frequent barker list.
2.) Greenies, Dentasticks by Pedigree, Marrow bones and Milkbones brand biscuits by milkbone - or if you are lucky and can afford holistic no grain dog snacks that remove tartar? Try those.
NOTHING at this point is going to get rid of the tartar by brushing. Someone here recommended the ropes. I am "iffy" with terrier breeds - and I don't know the particular type of ropes recommended so they may be just fine. I can only tell you with Pit Bull Terriers - THEY chew rope toys until they are demolished, knots undone, even on the toughest ropes and the sinews get swallowed, lodged in their intestines and cause the strings to wrap around the intestines and make blockages which lead to death and or surgery if you catch it in time. I have a doggie vacuum - Casper TWO TIME VETERINARIAN hospital CHAMPION! First time surgery - over $1200.00 and he swallowed his leather collar, with 3 metal tags (rabies, ID tag, and AKC scan tag), part of a milk jug we have NO idea how he'd gotten, and a plastic bag or something unidentifiable. The vet gave us back the tags, but kept the 22" collar. The Xray hung in the office for a long time with a note that said - "BUY OUR COASTAL NYLON COLLARS and save your dog from SURGERY!" The leather has salt in it - and the older dog chewed the collar like a rawhide while we were gone and when it was off? Casper ate it. Three days later, shivering, and sickly looking we rushed him to the vets. THEN two years ago for Christmas he was given a rubber duck. The interim vet said it was cancer - a mass. I nearly passed out. Casper is the other 1/2 of my heart. The girls in the office shot the xray picture thinking otherwise - to the Vet at a conference and in less than 1 hour - we rushed Casper to the vet for emergency surgery for removal of a rubber duck - at $800.00. (guess second surgeries are cheaper?)
We've had tongues sewn back on, jaws reattached, ears fixed, - our list goes on and on - (we also fostered for years) oh wait - we have that DOG----we still foster. And we trained for years, and showed for year, and when I had breast cancer surgery? (turned out not to be which was good) The doctor joked and said "When we opened you up you know what we found? Dog hair, no cancer - just dog hair." Okay great - everyone's a commedian. So I told him he was a boob. I don't think he liked my joke.
ANyway - with your little terrier? I'd try the marrow bones, and if you can call around and get a good price on a teeth cleaning? I'd have it done. It will make your dog feel better for certain. If you can't find any of those products in France call a reputable dog food company there and ask them what they recommend for tartar build up. But for sure you're safe with the bones - hip sockets especially.