If we're looking for an older language to be filled with words of power....I definitely think we're talking about Valarin (which Melian knows). Alas, *we* don't know it, so we are very limited in what we can say. Here is the entire corpus:
http://folk.uib.no/hnohf/valarin.htm
I have to imagine we can take individual words from this list and 'pepper' them throughout the Sindarin text. Alternatively, we can come up with one phrase in Valarin, and then repeat it as a refrain to Melian's song.
We can probably come up with something like banning everything that is Marred using those words....
mâchan supposedly means "authority, authoritative decision" (WJ:399)
dušamanûðân "marred" (WJ:401)
amanaišal "unmarred" (WJ:401)
Aþâraphelûn, supposedly meaning "appointed dwelling"
delgûmâ a Valarin word the exact meaning of which is not given. (WJ:399) It is, however, stated that it influenced Quenya
telumë "dome, (especially) dome of heaven" (LR:391 stem TEL, TELU), which was altered to
telluma "dome", especially applied to the "Dome of Varda" over Valinor; also used of the domes of the mansion of Manwë and Varda upon Taniquetil.
akašân supposedly means "He says" with reference to Eru; the source of Quenya
axan "law, rule, commandment". (WJ:399)
We could use 'delgûmâ' for the Girdle, as it is like a dome of protection and we don't know what that word actually means anyway.
There is not a good way of expressing 'keep out' with the limited vocabulary we have for Valarin. We also know practically nothing of the grammar, so trying to figure out how to change 'authority' to 'by my authority' or something is beyond what I know how to do. Nor do I know how to change 'He says' to 'I say'.