I have learned in accounting that companies can add any amount of
profit on their products as they want. That can be 10 - over 500 %.
They are not forbidden to do that. But unless they have a very strong monopolist situation, they will need to try to price their products at lower or similar prices than other companies.
I also think that international corporations like Coca Cola, Nestlé
definitely make more than 200 % profit for each of their products
(does anybody know how much).
I am not sure. They certainly make a huge amount of profits, but they also have a huge amount of capital.
Accounting is mostly about pretending to be transparent, while being as opaque as one can be. Companies report losses when they are making profits (in order to dodge taxes, for instance), and profits when they are losing money (to avoid bad publicity). So the answer to this is quite certainly "we don't know, unless we are in the board of one such company".
I don't know much about what is done with that profit, as every cost
the company has is already included in the price. That is not the
Part of it is distributed among shareholders, and part of it is reinvested.
My question is: What would happen if by law EVERY company on this
planet can only make a maximum profit of 10 % for each product. When
answering please ignore potential legal loopholes or anything similar.
They would artificially increase their expenses. Government doesn't allow them to have profits? Then they will pay higher salaries to their executives, higher rents to their landlords (and provide that their landlords are their relatives or people otherwise related to them). Are those loopholes? If so, the answer here is, "companies will find loopholes in order to keep their profits"; asking to keep loopholes out of the answer is more or less like asking "what would happen to someone who takes cyanide, but please ignore the toxic properties of cyanide"...
Prices would definitely decrease but what about wages for workers.
They would stay the same, because the profit doesn't affect them. Only
the wage for managers and CEOs would decrease. But what about
international economics. Would it remain stable.
Manager and executive officers salaries would most certainly rise. And there would strong incentives for them to reinvest part of those increased salaries in the companies. International economics would be stable, but politics would not: the incentives for corruption, money laundering, tax evasion, etc., would be enormous, and so would the political pressure to repeal such kind of legislation.
And the most important question: Can a law like that (if valid in
every country) actually work on long-term?
No, it cannot. First, Somewheristan will certainly realise that if they have a more generous law, they will attract the headquarters of international corporations, and consequently increase their tax revenues. So, as long as there are different polities with different legal systems, this cannot work. You would need either a world government, or a single national government capable of imposing its own laws upon the other (by violence or credible threat of violence, of course). And it is difficult to see what advantage would such a super-imperialist polity obtain by doing this.
Second, political power is very responsive to economic power. How many politicians would survive free elections if the private companies only donate to their adversaries? How many dictators can hold onto power if the capitalist class of their country opposes them?
Third, as accounting is about secrecy, you would need a government capable of actually controlling the internal workings of all relevant companies.
You would need a very strong worldwide government to do something like this - but a very strong government inclined to do this would be better off if it simply opted to superceed a market economy by something like "communism", a "gift economy", or similar. If you are going to fight against a giant, you should aim at killing it, not at shaving it.