Some great additional content here, I wouldn’t have thought of the stable diffusion trick— very clever!
Read this book, it's great! https://www.amazon.com/Fluent-Forever-Learn-Language-Forget/dp/0385348118
Find a spanish speaking girlfriend.
I'm lazy and haven't made progress, but IMO:
Highly recommend Fluent Forever
Do you have ANY intentions of becoming a permanent resident and/or citizen of France? If so, you may be better off becoming a European Union (EU) citizen, at least at first, starting off. I know that may not make any sense to you and you may be thinking "noooooooo!", but being an EU citizen actually is the great equalizer when you are in an EU country like France. This is because EU citizens have Freedom of Movement rights to live/work/retire anywhere in the European Union Countries (27 Countries), plus the European Economic Area (Iceland, Lichtenstein, and Norway--but Lichtenstein is allowed a person quota) and Switzerland (which is not part of the EU or EEA, but has treaties with the EU). However, in the case of retirement (being a pensioner/retiree/disabled), you have to show that you have "sufficient funds" to be able to support yourself.
But, as a US citizen (and in this case as a EU citizen too) with a software engineering degree with decent pay, this shouldn't be a big deal, as you will be paying US taxes for Medicare and Social Security (although if you make below US $104,000? you get a tax credit to where you basically effectively do not have to pay US taxes). You would also likely be eligible for a pension in the country you worked in Europe too. You would likely have enough income to live in anywhere in Europe (besides the city-states and perhaps Switzerland) with your US social security alone (again, as a full time software engineer with a degree from a US university).
What I am saying is that there are more booming places for software engineering, such as Ireland, which also pay better. You would be better off working there, getting Irish and EU citizenship (dual citizenship is permitted in Ireland), while learning French and brushing up on your skills there. Then you can move on to France and continue with your plans there, whether you want to become a permanent resident (which is much, much easier as an EU citizen, as it's not tied to your job at all, especially considering the unemployment in France is atrocious) or more.
Regardless, this particular old post will be useful for you. Basically, knowing languages (being a polyglot) and having a masters degree in Software Engineering likely will be super useful in increasing your prospects for long-term success. [The OP does get some flak because "immigrating to the EU has become harder 'since then'" but the information is still valuable, especially to somebody just starting out.]
As for language learning, I personally took this course, and I am now confident that I can learn even very difficult to learn languages. But, I want to point out that you definitely do not need to go overseas and do a "language immersion program" in a foreign country to become fluent in a language. Also, outside of the Anglosphere (English-speaking countries) it is very common to be able to fluently speak a second language, so learning a second language is definitely not "mission impossible"--even for native English speakers. You may find this recent thread useful. You also might find r/LanguageLearning and Language Learners' Forum to be useful too.
What really makes a big difference is having a very good language teacher (such as via Skype). For example, I am learning a difficult Slavic language. My teacher runs a ___________ Language School in the UK and has many years experience teaching at both _______________ Slavic language schools and French language schools. She is fluent in ___________ Slavic language and French and English. Anyways, for a 50 minute session via Skype it costs about USD $25, and it's definitely worth it as she's worth her weight in gold.
To practice speaking in French you would want to do a "virtual language immersion" via iTalki.com. Basically, you pay a native French speaker about $10-20/hour (depending on their rate) to speak with them the whole hour--with you not speaking a word of English. You can do this for even 4-6 hours a day, with different speakers, for your "language immersion program" to help you become fluent. But you do not want to use iTalki as a primary source for tutoring, as having a great language teacher really goes a far ways. iTalki basically exists because a lot of people are underemployed in the "Gig economy" and so they are just trying to get paid, rather than trying to bring out the best in you.
Glossika.com is highly useful in making you become conversational too.
You should also get the book "Fluent Forever", which was written by an engineer polyglot. Anyways, engineers (at least in the formal disciplines--I am saying this as an electrical engineering student) are highly efficient at devising quick learning methods and are also highly efficient in their work and skills too.
But, for getting started, this is what I would do.:
It should be noted that the happiness of immigrants very closely mirrored the happiness of natives in the World Happiness Report.
You should also make a notebook in Microsoft OneNote (or something similar) and have subsections for each "prospective country" that you may want to emigrate to. You can add various posts that you find valuable to each of the subsections so it's all easy to search and find.