Which route you take will depend on your longer-term plans. If you are planning to stay in Switzerland for a long time or permanently, then it's worth doing the local D-Schein (which is also valid on the Bodensee) if you want to sail anything with more than 15 m2 sail area. If you are only here for a couple of years or so, then I'd think twice about bothering with the D-Schein and focus on sailing yachts outside Switzerland.
In terms of international licences, the only ticket that is technically "international" is the ICC (International Certificate of Competence), which is valid in any country that is a signatory to resolution 40 of the UNECE, more info here: http://www.rya.org.uk/infoadvice/boa...Pages/icc.aspx
It can also be endorsed for inland waterways with an extra online CEVNI test. Depending on your nationality, most national governing bodies will issue an ICC on request if you hold or complete a minimum level of their own qualifications (Day Skipper in the case of the RYA). The ICC can also be taken as a stand-alone course/qualification.
If you are looking to charter in the Med, for instance, then I would advise you to look at a qualification of ICC level or above from a national governing body (the RYA's qualifications are probably the most widely recognised in the world, regardless of ICC).
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For e.g. chartering on the Med, any internationally recognised license is fine.
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... depending on the charter company, so check before you book!
When doing your research and choosing your course, do check with the issuing body whether you can join their scheme at any level (as is possible with the RYA) or whether you need to start from the bottom and do all the qualifications in sequence regardless of prior experience.
Unfortunately, with a few exceptions that don't seem to apply to you, the Swiss system is basically incompatible with any other country's system. In other words, if you live here and do a foreign ticket, it's not valid on Swiss waterways; even with a commercially-endorsed Yachtmaster ticket and instructor qualifications, I had to the D-Schein from scratch here. Likewise the Swiss D-Schein is not valid on the sea, you would have to progress to the Swiss Hochseeschein (formerly known as the B-Schein) which is the Swiss qualification that confers an ICC, and there are other ICC-level qualifications from other governing bodies with far less onerous requirements.
However it's not all bad news; as I mentioned, a licence is required in Switzerland to sail boats with more than 15 m2 sail area (official measurement, not manufacturer's spec). The corollary is that if you have 15 m2 or less, no licence is required. That's why, Sail YSL, we use dinghies for our work in Switzerland.
It's a bit of a minefield, as you can see!