Continue learning after Duolingo
Duolingo is fun and good for getting started learning a language, but it won't get you a job. I don't think using Duolingo can get you past the A1 level of CEFR, maybe A2 for reading. You can of course mention it on your resume as a hobby.
How to continue learning and eventually earn a certificate? Where I live I can take language lessons, but I will not get a certificate that can be mapped to CEFR even though I live in the EU. The way the lessons are organized, it is not possible for me to attend them. Can you learn a language online and eventually earn a certificate? A certificate that can be mapped to CEFR? It doesn't have to be free, but they have to offer conversational lessons.
I have found https://www.frantastique.com for French, it is even mentioned on the CEFR wikipedia page, but it doesn't offer conversational lessons. I also found https://www.lingoda.com and they claim they offer CEFR certificates for English, French, German and Spanish.
I would like to get tested and get a certificate for English and French, even if I have to study for a couple of months or a year first. Any recommendations?
For becoming more fluent, I recommend using a variety of resources. I personally use podcasts, tv shows, music, a grammar book, and speaking (message and Skype) with native speakers. All of this along with Duo has really helped me with reading, writing, and speaking. And don't wait till after you've finished your tree! I started out with many resources at the same time as Duo, and that approach has been super helpful! You'll notice vocab and grammar overlapping on the various things you use to learn, and it really satisfying to be able to already have that foundation from one resource when you encounter that area on another. And don't let the fluency/levels on Duo get to you, they don't really matter in the real world. Just keep practicing every day and you'll easily be able to see for yourself how fluent you really are.
Regarding CEFR certificates: there's no general requirement to study at the institution that awards the certificate. It's perfectly possible to organize your own preparation for a CEFR certificate (by mail-ordering textbooks, taking language classes, hiring a tutor, using online tutors via video conferencing, taking online courses like the ones you linked to, etc.), then visit a test centre in another city or country to actually sit the examination. As far as I know there's no way to get a CEFR qualification purely online: the examination must be taken in person at an official test centre.
I don't know anything about certification, but for post-Duolingo study I can't recommend Clozemaster enough (https://www.clozemaster.com). It's got a lot of wonderful features (almost all of them available for free users) and if you pay for premium you get some even better features - such as my favorite combination of listening practices with typing and reveal-after-answer translation.
Overall great for continuing to expand your vocabulary, and on a plus side, it's cheaper than Duolingo Plus!