How to speak a language with a third declension adjective?

Question

Most Latin adjectives related to names of countries and languages are of first and second declension: Latinus, Graecus, Anglicus… If I want to express that I speak in any such language, I will simply use the derived adverb: Latine/Graece/Anglice loquor.

But how does it work if the adjective belongs to the third declension? The list in the linked question contains, for example, Islandiensis, Melitensis, and Thailandiensis. If I want to express speaking or otherwise using these languages, do I use the adverb normally? Saying Islandiensiter/Melitensiter/Thailandiensiter loquor sounds weird. I have never seen such adjectives in such use, so I have no intuition. (I don't know whether the Romans ever had third declension adjectives together with lingua, so there may or may not be classical help.)

Is the -iter ending used under such circumstances? Is a first and second declension alternative always used instead (Islandice, Melitice etc.)? Or should I simply use the ablative like lingua Melitensi loquor?

This may be a hard question to answer, so answers based on experienced intuition are also welcome. To decisively settle the matter, I imagine one would have to give examples of use, clear avoidance (where the adverb is used for first and second declension but not third), or using first and second declension alternatives for this purpose but third declension elsewhere. I understand that is much to ask for, so I do not expect airtight proof.

Answer 1

Indeed, the -ensiter adverbs are not an option. As shown below, either a language actually has (or also has) a first/second declension adjective and one may use the corresponding adverb, or only a third declension one is possible, in which case one has to use the ablative with the noun lingua.

• Icelandic is lingua Islandica: the Latin Wikipedia page gives Conradus Gesnerus, Mithridates: de differentiis linguarum (1555) as a source. It can (not easily) be read here. Ludovico Antonio Muratori speaks of Islandica Lingua in his Antiquitates Italicae Medii Aevi (1738-43), see here. And here's what appears to be a lexicon of Icelandic (1959): Lingua Islandica: Íslenzk tunga:

  

  The adverb Islandice is also attested: see the front cover of a 17th century work (below) and the title of a 2009 book, Clari Fabella: Islandice et Latine, by Gustaf Cederschiöld, Samuel Cavallin.

  

• Maybe by analogy, being the only official and national language of Thailandia, Thai is often seen on Latin websites as Lingua Thailandica. For example, it's in Vicipaedia's Linguarum officialium catalogus. However that's pseudo-Latin at best, apparently absent in any official document. Instead, the Acta Apostolicae Sedis (January 1983, Vol. LXXV, Pars I) reports the following:

  Liturgia verbi expleta, Summus Pontifex, post audita verba dicta a Beatissimo Domino Antonio Petro Kòraiche, Patriarcha Antiocheno Maronitarum, ex noviter creatis primo, hanc allocutionem habuit, adhibita lingua : Italica, Gallica, Anglica, Hispanica, Germanica, Lusitana, Thailandiensi, Lettonica, Croata, Polonica [...]

  It seems clear that if "Thailandica" had been a possible choice, the editor would have used it. However, unlike Islandice, Thailandiensiter is not found by a Google verbatim search. Thus, apparently the way to go here is lingua Thailandiensi loqui.

• The same seems to be true for Maltese. The adjective is indeed Melitensis (not "Meliticus/a/um"), it can be found in Cicero's In Verrem, and did not change in modern times. Mikiel Anton Vassalli published a lexicon Melitense-Latino-Italum (it should've been Latinum, I think) in 1796:

  

  Inside, the abbreviations "It." and "Lat." are used for Italice and Latine, but Melitensiter is nowhere to be found, and there are a couple of in Melitensi sermone that really seem to substitute the adverb. I believe this is enough evidence to conclude the ablative is necessary to "speak Maltese" too.

• For Chinese there is the problem of distinguishing the group of related language varieties, and what is commonly referred to, i.e. Standard Mandarin. The former appears to be described as lingua(e) Sinica(e), the latter by lingua Sinensis - see the Vicipaedia discussion, but in practice there may be no difference. The Latin Wikipedia page references Lingua Sinica by Petrus Ribadenaira, Philippus Alegambe, Nathaniel Sotvellus, Bibliotheca Scriptorum Societatis Iesu, vol. 2 (1676), p. 598. Besides that, Frédéric Guillaume Bergmann speaks of lingua sinica and scriptura sinica in his De linguarum origine atque natura: dissertatio philosophica (1839). On the other hand one has Martino Martini's Grammatica Linguae Sinensis (1651-1653), which deals with Mandarin specifically. At any rate, as was to be expected, Sinensiter loqui is not attested, whereas Sinice loqui is.

• Irish is lingua Hibernica (as seen in the title of the article "Noticulae De Lingua Hibernica", by Vernam Hull (1954)) and Hibernice loqui is even attested

• Dutch is lingua Nederlandica, attested in Humanistica Lovaniensia - Volume XLIII, Corona Martiniana. This is also the expression used by the Belgian Schola Nova. And indeed, the adverb is present e.g. in de Groot's Lineamenta theologiae christianae universae.

Answer 2

I never heard of using an adverbial form of a nation or an ethnicity to refer to its language and speaking in it. Usually, it is:

"Linguā Latinā loquī"
to speak by means of the Latin language...

as in the ablative adjectival phrase.

In your case,

Iceland = Islandia, and Icelandic= Islandiēnsis;
Malta = Melitae, and Maltese = Melitensis;
and Thailand = Thailandia, and Thai = Thailandiēnsis¹

Therefore (assuming these adjectives are I-stem):

Linguā Islandiēnsī loquī = to speak (by means of/in the) Icelandic (language)
Linguā Melitensī loquī = to speak (by means of/in the) Maltese (language)
Linguā Thailandiēnsī loquī = to speak (by means of/in the) Thai (language).

1. Joonas Ilmavirta, response Aug 6, 2016 to "What are the Latin names for modern countries?" by Sam K, Aug 6, 2016, (accessed May 6, 2019), https://latin.stackexchange.com/a/1330/4731; "List of Latin names of countries," Wikipedia, last edited May 6, 2019, (accessed May 6, 2019), https://en.wikipedia.org/wiki/List_of_Latin_names_of_countries.