Let's look at the easiest languages

 Navajo

Can I see in Duolingo how many lessons?

At the end of level six it sets a test to see if you can jump. Unless you jump you cannot see how many lessons are ahead. Can I cheat the system by using google translate to fill in correct answers. (This would speed me up, and I would still be learning.) No. Google translate has dozens of languages, the common ones and obscure ones, but not Navaho.

Google Translate

This is ridiculous. I could add half a dozen words to a dictionary. Would that be valid? 

No. Because I am not a native speaker. But it could be in a not yet published system waiting to be checked by native speakers.

Google translate's algorithms would check whether my answers are correct. At one point I was invited to join an online panel correcting English translations on Google. They ask a dozen people who have signed up as native speakers (passing a spelling and grammar test), and when all agree they add the result.

Esperanto 

Esperanto is another easy language. Easy for some. it has regular rules. No exceptions. But you have to learn them. i don't find them easy to remember or say. Easy to read. And lots of speakers. Used by Hungarians. 

Interlingua

I like Interlingua. But nobody speaks it or uses it. It's what you use if you are a polyglot in a desperate situation, like I was trying to warn a group of laughing Spanish girls that they needed to evacuate Euston station in London in the year 2000 because of a bomb scare. You picks words which are the same in several languages and add the most common endings, such as s for plurals, o for male, a for female. Similar to Spanish.

I like this because it says what it is. I think every flag should have its name in English or at very least in its own country's major language.

Interlingua seems to use salute for hello, saluta, for say hello, bon die for hello/ good day, salute mondo for hello world.

Useful Websites

https://en.wikipedia.org/wiki/Union_Mundial_pro_Interlingua

Translation English to Interlingua

https://glosbe.com/en/ia/hello

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