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EDIT: can someone advise me a license? I have no problem if other people take this grf as a basis for their own work as long as I'm credited. I probably need to supply a license file and the source too then, right?
Well. It depends, I'd suggest GPL v2+ or CC-BY or CC-BY-SA. While GPL requires others to also use GPL and it requires you to provide the source (you could just zip the sources and attach it here), the CC-license wouldn't require that of you. Both (or all three) licenses require others to give you credit for your original work. I'd advocate for either GPL v2+ (need to provide source) or CC-BY (no need to provide source) as both are compatible with OpenTTD's GPL v2 license; I'm not 100% sure about the compatibility of CC-BY-SA with the GPL v2.
I'd advise explicitly against any non-commercial clause as that'd render it incompatible with the GPL and also prohibits people to put that NewGRF along with OpenTTD on e.g. a DVD which comes along with a gaming magazine and alike (that already counts as commercial use).
You're welcome, of course, to request a DutchTownName project on the DevZone. Using that basically fulfills the criterion to provide sources, but it'd also require you to get familiar with the use of the version control system mercurial (not a big deal, you have python already, so it should be quite straight forward as it's also python-based, but well...)