There are two factors at play here:
- Will international users understand non-localized numbers and dates.
- Will international users be put-off by your non-localized site and not want to purchase your product.
Given your statement "I don't want to invest a lot of time on detail before it has gained any traction," the best option is probably to just go with that is standard in your main target market, and let others figure it out. Most people will still be able to understand what you mean as in (1) above, but might be less likely to buy from a foreign website. Since you don't have any customers at all right now, the there are probably bigger opportunities for improving your product elsewhere, and you can worry about localization after you get traction (especially because you don't yet know which localities will be important).