"Informal" conveys no clear meaning (to me) in this context.

Will "a not legally accepted" make sense?

It would tell me something, and there's nothing wrong with the grammar; but it would drive me into "interpretation mode": what exactly does he mean?

Your English seems to be a translation of someone else's Hebrew; and I suppose s/he uses a Hebrew word that commonly means "informal." What do you think s/he meant by it?

When I think of Esperanto, I think of constructed or invented. Then I stop thinking. What else is there to say? Well, it's not the language of a people, and (I'm guessing) it's not a "legal language" anywhere. "Legal language" means "official language"; and "informal" shares meaning with "unofficial."

"An unofficial, constructed language" is not (to me) entirely satisfactory. What I'm thinking would be natural is a bit of a circumlocution: something like "Esperanto is a constructed language, not officially established anywhere."

"not officially established anywhere" is a correct definition but too remote as a translation.
To say only "unofficial" is neither precise, as some institutions might adopt it.

If you put "scare quotes" around informal, then I should read it as "if you have to do some figuring and guessing here, that's the way it is."

Thus: "Esperanto is an 'informal', constructed language." Note the comma between informal and constructed--outside the quotes, as you prefer.

Accepted, thanks.