Language is a concept which is, to a certain extent, foreign to Landian thinking. It refers to an entire set of words used by a given people, and as explained in the Book of Connor and Brigid, there are many worlds inhabited by alien races, each of which has at least one native language (some races, such as that of Earth, have many different languages). On the Land, ever since our world was first created, people have essentially spoken a single language (see next paragraph), albeit with the inclusion of various subwords from alien languages. It is therefore difficult for us to understand how there can even be separate languages, at least on a single planet. That is, it seems as if all words should simply be understood by everyone, or at least, the majority of words should be (no one is likely to learn every word in their own language). Even within a single language, there are often synonyms, different words that mean essentially the same thing. So in a sense, different languages are rather like extended sets of synonyms. In any event, it is unclear whether there might ever be different languages spoken on the Land by different groups of people, though upon the founding of the Land's second country in 913, there has been some speculation that it, or perhaps any subsequent countries that may arise in the future, might decide to adopt their own languages, distinct from the one currently spoken by everyone in the world. This, however, seems highly unlikely.
Our language, in the rare instances that it is referred to by name at all, is called "Landish." It has always been understood to have been naturally imbued by God within the consciousness of the first people on the Land (though of course anyone born since then has had to learn to speak and understand language in the natural way), consists primarily of the same set of words known on Earth as "American English," in the form it was spoken on Earth at the time The Land was created, in the Terran year C.E. 1997. However, it is believed that some words of the original Landish had their origins in other of Earth's languages besides English, and that many of the subwords which have become a part of our world's language in the centuries since then- though not all are from Earth, the majority seem to be- are likewise from any number of different Terran languages and dialects, including "British English" (it's also worth noting that there are some differences in spelling between the American and British dialects, and that written Landish spells some words the American way and some the British way, for no readily apparent reason). Moreover, there is a concept on Earth of slang; that is, words which are not "officially" recognized as part of the formal language, but which are generally widely enough spoken as to be understood by most people, anyway. On the Land, we rarely make such distinctions; words are words, and all words are of equal value. (Though it should be said as well that different individuals may have different styles of speaking, possibly owing to the relative isolation of villages prior to the Coming of the Order, as well in some cases owing to differences in social class. Mostly, it is simply a matter of personal taste, and of course a matter of which words the individual happens to be familiar with.) Regardless of whether subwords were considered slang or formal on their world of origin, and regardless of what worlds or languages those words, formal or not, came from, once a word comes into wide use on the Land, it is thereafter considered officially a part of Landish. (On the surface this might seem like another reason new languages couldn't come into being in other countries on the Land, though that's not actually the case. Words that are added to our language normally are introduced individually, whereas an attempt to introduce an entire new language at once would require a substantial concerted effort of teaching and learning among a given group of people.)
It should be noted that language need not be vocal in nature. For example, merfolk use something called "sign language," in which hands are used to form different shapes to represent words or letters. During the project to create merfolk, this form of communication was taught by spirits to the Sorreters involved, who later taught it to the first generation of merfolk. Sign language was originally developed on Earth, apparently for the benefit of people who were deaf and/or mute, disabilities which thus far have not been recorded among any of the Land's people. (Merfolk are also capable of vocally speaking Landish; sign language is only used when they are under water, which prevents them from effectively vocalizing, and which more importantly requires them to use their gills to breathe, while mostly keeping their mouths and noses closed, to prevent drowning).