"My product uses open source and so internationalization requirements don't apply."
Tell that to your customers when your software doesn't work for them.
A product is only as good as its weakest component. If you use open source in your product and it isn't internationalized, then it may be that your entire product can't handle international data. For example, say you base your product on one of the Linux flavors that isn't internationalized, and say your primary market is, oh, China (I hope a certain VP is reading this). Does that make sense? Why would Chinese customers run on a platform that doesn't support Chinese? Would you run on a platform that doesn't support English? My German is pretty near fluent, but I still run on an English platform. Even if I switch the interface to German, which I sometimes do, I make sure that it can process English correctly.
When producing a software product, all customer requirements should be considered. And whether you write your own code or pull it from open source, those requirements still count. If your market is worldwide, or even within the EU, internationalization is not a "nice-to-have", it's a must. So when choosing external components, be they from a vendor or from open source, consider the work it will take to get the internationalization up to snuff. Then make your decision.
