In the US and Canada nutrition counseling is not well regulated. So while technically you could offer consultations there, your voice will be competing with every person calling themselves a “nutritionist” or scam product and fad diet that purports to balance ph levels, clear up skin, or reduce inflammation. Both the USA and Canada have regulated programs for “dietitians”. Dietitians are required to go through a formally recognized education program. However, they only seem to be employed in clinical settings, such as hospitals or nursing homes.
When you market yourself, you have to provide some kind of expertise, so if you happen to lack some certificates that makes for example a nutritionist in another country “more legit”, it might just be a disadvantage in competition, but as long as you don’t be deceptive about it, customers are free to choose to book you or not.
So it’s less about the formality, but how to market & sell your service on a competitive and over saturated market.
Where/ in which country you would pay taxes? Most (!) likely depends on country of residence connected to a bunch of laws in great deal connected to permits and also to your clients/ employers applicable to them?
The insurance situation makes it even trickier to be competitive. As an out of country student, there is a 0% chance you’re going to be able to bill insurance for it, so you’re competing against someone going to a local licensed (in some cases, at least) professional for $20-$50 in most cases. My insurance plan covers up to 5 dietician visits per year as preventative care, meaning $0 out of pocket for them.