Ash

Spero tibi librum placere

Familia Romana is the first book. https://archive.org/details/llpsi-pars-i-familia-romana

Its hard to choose, interlingua, esperanto, lingwa de planeta, ido, uropi, lingua franca nova.
Personally I think its better to develop an international sign language.
It's much easier to learn as you dont need to learn new spellings or alphabets (the sign for "a" can also be greek alpha for instance), or germanic or slavic or latin roots, it is only actions added to your own local word, so literally verbal monoglots can speak to any other monoglot. No matter how thick their accent is.
Plus it would greatly expand the deaf communities international reach, would not exclude the deaf, and I like the idea of children learning an international auxiliary language that can also help the hard of hearing.
And if you've ever been in a loud bar or nightclub or restaurant in a foreign country in europe it would make ordering much easier. Obviously unfortunately it does somewhat exclude disabled people with reduced mobility and wouldn't work on the phone, but we all have videocalls now and we shouldn't be on the phone while driving anyway. Also it's quite fun. I've started in French Sign Language (Elix app on ios app store) and am pushing to have lessons for my colleagues as we work in medical training and I think its nuts its not offered let alone encouraged or compulsory.

So I say we should develop Euro Sign Language.

I mean we all use the middle finger already, lets expand on that.

The reintroduction of Welsh via primary schooling in Wales has been super successful, a really good example of how quickly kids learn and I used to know some couples who had young children that could speak Welsh and they didn't and it really bugged them as they had no idea what their kids are saying. But pretty much in one or two generations of children, a very endangered language is coming back.

In that case read Lingua Latina per se Illustrata by Hans Orberg. I now read quite well in latin thanks to his self teach books, but theyre really interesting - they teach using the natural method... ie there is no english or french or anything - the entire book is in latin. The first lines are :

Roma in Italia est.
Italia in Europa est.
Italia et Francia in Europa sunt.

And then you have to figure out (with the help of cryptic annotations) that est (is) is singular and sunt (are) is plural. No going back to your own language, no phrases. By the end of the first chapter you are beginning to decline nouns and adjectives. Puella pulchra est, Puellae pulchrae sunt. And you have no idea you are doing it. No boring rote learning of declensions, its like you're learning as you would as a child.

And now I can mostly understand reading romance languages I don't know well(romanian, spanish, etc) thanks to learning latin (Also, I am a fluent french speaker) and other people have copied his method into loads of languages so I collected lots of natural method books for german and italian etc.

kDrive - infomaniak - Swiss.

The crazy thing is the apps are just browsers (edge for web, electron for app) and it still sucks. And the open source bits work well, but anywhere theres closed source (onedrive) it all goes to shit.