Is it allowed to carry seeds on an intercontinental flight?

If your destination is an EU country, it's worth looking at the UK rules for plant material (including seeds), which are stated here. As the UK is (for now) still in the EU, these are EU-wide rules.

Under "Personal Allowance" it says that you can only bring in seeds in your personal baggage from other EU countries plus Switzerland, Algeria, Egypt, Israel, Jordan, Lebanon, Libya, Morocco, Syria, Tunisia and Turkey.

The corresponding French rules are here. My French is far from fluent but as would be expected the French rules look at least superficially pretty similar to the UK ones.

Coming from South America, if you want to be sure of staying within the letter of the law I would play it safe and not bring any plant material.

YOU MUST DECLARE IT AND HAVE IT EXAMINED

To repeat: YOU MUST DECLARE IT AND HAVE IT EXAMINED

Please read the entire Answer carefully, I am in not suggesting OP smuggle or break the law in any way. I am also not going to provide references to every country's customs and agriculture rules.

It is not the case that plant materially is unconditionally forbidden, however, this is something that Customs and Agriculture Officials at your destination will have to determine.

Meaning, you can carry plant materials to the border, but upon arrival YOU MUST DECLARE IT AND HAVE IT EXAMINED.

If you declare it and it's not allowed, it will simply be confiscated and destroyed (provided it's not a dangerous or otherwise illegal substance). If you are caught smuggling it, it's a significant fine at the least.