While I don’t have any kids who went the engineering route, all three went from Pre-K through some portion of elementary at a French immersion school. My eldest is fluent and loves visiting France and finding opportunities to use the language. My middle and youngest have little interest in French.
However, middle and youngest are at least proficient in French, and some people might consider them fluent. Eldest took the AP sophomore year, middle took it junior year and youngest dropped French freshman year (last full year of any French speaking class was 3rd grade).
All three have had occasions to speak French recently. For example, middle kid is a grad student, and makes some extra money as a server at a Mexican restaurant (and upscale tequila bar). She had a table of folks visiting from France and they told her up front that their English was not great. She spoke French the whole time, and at the end of the meal, they said her accent and grammar were perfect. So 10+ years removed from AP French, she still is fluent/near fluent.
This is a long winded way to get to my point - I do not think any of my kids would have done well, or nearly as well as they did, in undergrad if they were immersed in college classes in French. There is too much vocabulary in a foreign language, specific to a major, that you just don’t use in regular conversation.
Would they know those engineering terms in English? Very likely, especially with two engineer parents. Would they recognize them in French? Doubtful. And trying to navigate college level material in an already rigorous major, then throw in the foreign language - I think that could be very difficult. I would not want to experiment with my kids to see if it worked out. Better to do a semester or year in France.
One program that has been around for a while is at University of Rhode Island. They have a well established year long study abroad for engineering students. Students intern for an engineering company and take courses to improve their language skills.