“Even in countries where their ideology has meant that it is used for precisely that (social enginering), with probably France being the best example, they've never stuck with it in reality, because it doesn't work. The Foreign Legion is I believe still not allowed to enter French soil (I think anyway).”
The French Revolution for military use imposed Compulsory Military service and the Levée en Masse was quite successful in providing the young Republic of enough soldiers to won against the European Monarchies and to crush internal fights.
As a tool for social engeneering it did work e.g. imposing French to all yong male French, and in mixing all origin and socila backgrounds. If you can give me a exemple of a failure in the French Society that can be let on the Service National, thanks.
It was cancelled because it is not cost effective anymore.

And perhaps it is new to you, but France has no more Colonies. So the Foreign Legion is based in France.