I'm curious, what language would you consider being easy to learn
A language with no grammatical irregularities for starters. And one where the phonetics are consistent. Constructed languages can offer this. Whether any existing ones are sufficiently easy, I'm not sure.
And then some mechanisms that facilitates vocabulary building. For instance, I like the affixes in Esperanto, as understanding the root word and then the affixes allows you to pick up all kinds of words you never explicitly learned. And example is -ejo, which indicates a place, could be combined with a root word such as the verb forĝas (to forge, root: forĝ-), yielding forĝejo = place where one forges. Or monero (money, root: moner-) + -ejo yields monerejo = place where one stores money (= monero).
I'm sure with modern linguistic knowledge a much easier language than Esperanto could be constructed.
However, it's not that you can dictate a language
The question was whether an auxillary language would be a good idea. It would necessarily be dictated. Every citizen would learn it in school. The proposed benefit having a a common language easily learned and spoken equally well by all member state citizens, that could be used to cross language barriers (like English is today), and that could be used within EU (i.e. all institutions) as an official language.
For the record, I am intrigued by the idea, but I am very open to this being a bad idea, which is why I made the thread to hear people's opinions.