"Regular Contract Bridge" doesn't really have a meaning, especially when it comes to Alerting. In rubber bridge, you usually are playing cut-for-partners, so the system is specified by the Club (what happens at informal rubber is up to the players). In duplicate, L40A1b assigns how players make their system known to the opponents to the Regulating Authority. So any question on Alerting depends on your RA.
As you said, the Dutch use pretty much the World Bridge Federation rules (note that these are not "what the WBF tells people to use", these are "what is in force for games run specifically by the WBF"; i.e. where the WBF itself is the RA). Those are really designed for high-level play with screens, and say a lot of "Alert what should be Alertable"; which works for their players, maybe not so much at a regular club. But it works for the NBB, so what do I know? Under those regulations, as you also said, you should Alert it as it is uncommon enough to be unexpected.
Side note: "Alert what the opponents are likely not to understand" leads to a lot of Alerting, which in face-to-face play leads to a lot of Unauthorized Information passed by partnerships, even if it is just "I remember our system, partner". With screens, each player Alerts their screenmate, without passing information to the other side of the screen, so "overAlerting" is safe. This also applies online with self-Alerts, which is why most RAs have an "if online, explain anything that you think should be explained".
Most of the answers here use the ACBL as the RA. If you're not playing ACBL events (North America, or the online NABCs), that's only relevant as an example of how "where you play matters", as opposed to "there's a global answer". However ACBL regulations are relevant to a lot of this stack's players, and the Alert Procedure has changed for 2021, unfortunately invalidating the other answers. Here's how it works as of January 2021:
The system is now pre-Alertable (because 1C is not Natural to the definitions); at the beginning of the round, you will explain that your 1C is "clubs or balanced". It's probably best in the pre-Alert to also make clear that 1D is "always unbalanced". Note: you should know whether for you, 2=2=5=4, or 5D332, 6D322 or 7D222 are balanced or not, and if it's quick to explain, then do that here too.
Your Announcement for 1C is now "could be 2" (which is what all the "could be short" people were doing anyway). If asked, of course, you explain "real clubs or balanced 15-19", with explanations of what is balanced again if asked.
1D is not in the list of Alertable Natural bids. However, the Alert Procedures do not make it wrong to Alert something that "feels wrong not to Alert" as long as it's not on a list of "do not Alert" calls. I would feel uncomfortable having not either Pre-Alerted it (although it is technically not Pre-Alertable) or Alerting it when bid (although it is technically not a required Alert). My feeling is that one is sufficient, and the pre-Alert is better. Again, if asked, you explain "5+ or 4D441, always unbalanced" (again, with your definition of balanced).
Online (Appendix O), you explain when you bid, whether it's Alertable or not: "You are encouraged to explain calls even if those calls do not require alerts." Pre-Alert is still required.
Finally, with all RAs, remember that the primary goal is L40B1a: "Each partnership has a duty to make its partnership understandings available to its opponents" (my emphasis). You are required to do it the way the RA requires, but using the technical letter of the regulations to minimize disclosure is a violation of the Law.