Tony Harris wrote:
> The Latin is a take-off of the Biblical phrase "Fiat
> lux!" in Genesis 1:3, which is literally "Be-made light!"
> and is the passive present subjunctive of "Facere" (to
> make) I think. 

"fiat" _may_ serve as the passive present subjunctive of 
"facere"; but the verb "fieri", of which "fiat" is the 
present subjunctive, also means "to become", "to come into 

> It is taken to mean "Let there be light!"
> and is from the Hebrew "Y'hīy ōr!" which is perhaps more
> literally "Be light!" as in "Let light be being in
> existance" more or less.  

...which is what I've always understood the Latin "Fiat lux" 
to mean - "Let light come into existence!"

> In the Esperanto Bible, that's
> actually translated as "Estu lumo!" so perhaps actually
> "Estu lingvo!" might even be the best translation, even
> if in our conlanging case "Fariĝu lingvo!" might be more
> accurate.

Wouldn't "Estu lingvo!" be more accurate?

Another well-known "fiat" comes in the Lord's Prayer:
Fiat voluntas tua - Thy will be done (Matt. 6:10)

Both there and in the "Let there be light" verse from 
Genesis, the Greek Bible has γενηθήτω which is the 3rd 
singular aorist [i.e. perfective] imperative of γίγνεσθαι 
"to become", "to come into being/existence." So the TAKE (το 
άνευ κλίσι Ελληνικό) version  will have ίθι γένε.

OK - here are phrases in TAKE.

Fiat lingua
ίθι γένε γλώσσο

τεχνητό γλώσσο

-friendly greeting_
(Literally "rejoice", just as in ancient & Koine Greek).

"Ein Kopf, der auf seine eigene Kosten denkt,
wird immer Eingriffe in die Sprache thun."
[J.G. Hamann, 1760]
"A mind that thinks at its own expense
will always interfere with language".