Hello, Thomas. Don't worry; your English is fine.
Worksman bikes, in stock form, take 1" threaded forks and a special kind of 1" threaded headset. Your mechanic is correct; the headtube of the
frame is unusual, but the
fork is basically normal. (It does have a narrow ID, and takes 21.1mm stems instead of the more typical 22.2mm).
Basically, the Worksman headtube has something like a 32.7mm ID. Normal 1" headtubes are 30.0-30.2mm. "BMX" 1" headsets tend
to be 32.5mm OD. 1.125" headtubes take a 34.0mm OD cup. The Wald 4080 is about 33.0mm at the nubs, and is soft enough to easily deform or file down to fit a worksman.
If you're hoping to run a 1" threaded fork, the easy option is to run a Wald 4080; it will fit, but you may need to file the 3 little nubs a VERY small amount. The next easiest option is to get a "bmx" sized 1" threaded headset, like the Tioga Beartrap. These will need some shimming, but you can do that by cutting up an empty can of moosehead. I kind of like running the tioga beartrap guts and nuts within the original Worksman cups. Links:
http://www.amazon.com/Wald-Headset-4080-Threaded-Plated/dp/B000AO5FPG/ref=sr_1_1?ie=UTF8&qid=1377535827&sr=8-1&keywords=wald 4080
http://www.amazon.com/Tioga-Beartrap-Headset-Threaded-Chrome/dp/B000C14N4Q/ref=sr_1_1?ie=UTF8&qid=1377536198&sr=8-1&keywords=tioga beartrap
If you're going to run a 1.125" fork, get one of these:
http://www.genuinebicycleproducts.com/parts.html (Scroll halfway down the page...)
I got one; very nice thing, for what it is. I haven't test-fit it yet, but I put one of the cups up against an OEM worksman headcup, and the OD matched very nearly perfectly. It was the same exact size, to my eye, but I bet a micrometer may show a very small (.1mm) difference ...
**EDIT**The Retro Ryder fits loose in a Worksman Frame, and would need to be shimmed. I found an older, very UGLY YST CS-568 headset where all of the guts fit nicely with the oem Worksman cups. The caged bearings drop in perfectly, and the rest goes into place from there. HTH... **EDIT OVER**
You can also mix-n-match bits from various 1.125" headsets within the stock Worksman cups, but in my opinion, this is more annoying than just dropping the $45 on the ready-made one from Genuine Bicycle Products.
I hope this helps; I tried to take the fact that you're a French-speaker into account and avoided slang and esoteric terms, but that's tough to do while discussing bike parts. By the way, "ID" stands for "inner diameter", and "OD" stands for "outer diameter."
Regards,
Rob