When and how can I find an external investor?

There are some angel investors that may invest in your business, depending on how much traction you have but most investors want to see a proof of concept and some traction. The best advice I can give you is try to get the app done and continue to have conversations with investors outside of your family and friends. All you need is to find someone who believes in your vision. Even if you don't find an investor who is willing to take the risk, the worst case scenario us you build some good relationships that can be leverage later and you will get some valuable feedback on your product.

Good luck.

Unfortunately, this is quite far from a likely investment. Several big issues:

(1) It requires a great deal more work (read: "time+money") to SELL an app than to MAKE one. The primary risk investors need to overcome is not the question of whether the app can be built... it's whether the app can be SOLD. So if you haven't been able to demonstrate sales traction yet, then this is an unlikely investment.

(2) When an investor is in the same country, it's much easier to enforce the law. If someone in the US were, for instance, to transfer funds to a Ghanaian bank, and then you were to simply walk off with the money, they would have very little recourse. The idea of interacting with an overseas court system is a nearly unthinkable headache. I don't mean to offend or accuse you of bad intent — just pointing out that investors have enough fear within their own countries; it's compounded when looking at a foreign opportunity.

My advice is that you'll need to find a way to finish the Minimum Viable Product on your own steam, show some sales, and THEN pursue investors.

If the opportunity is so strong, for instance, perhaps you can convince your Serbian colleague to invest his own time in completing the project — in exchange for shares in the enterprise? If you hope to convince an investor to give you cash, then a good "baby step" would be to first convince a developer to give you their time...

Happy to discuss further on a call, if you like.

Generally speaking, yes, it's possible to get money from USA investors even though your company is not local.

On the personal note, I am glad to see that all your programmers are from Serbia, as that's my home country and I live in the US now.

The app does not have to be done at all. 60% is good enough to test the market, so I would shift my focus on product--market fit validation, customer acquisition process and a business model with an emphasis on revenue potential, addressable market size, total market size.

Seed investors will put their money into pre-revenue companies, but venture capitalist, Round A are less likely to do so which makes bootstrapping incredibly important.
Here's my paper that can help you understand where does your company fall on investment readiness spectrum.
How do you know that #startup is ready for venture capital? Industry fit, $1B+ market size, big competitive advantage http://wp.me/pj2e9-by

Finally, I would be more than happy to work with you on your investor's pitch and be ready before your colleague starts making connections in SF:
http://www.anagard.com/services/InvestorReadyConsulting.html

I have 25 years of experience working with early stage technology companies and investors.

I’m often asked about fundraising strategies for VC funds and angel investors. After raising capital and exiting from multiple startups and investing through 15 venture funds and dozens of angel investments I have seen thousands of deals.

I’ve found that the most productive use of time for both of us is scheduling a call through my profile.