Really, the D&D languages are, for the most part, merely labels. There is no listed limitation nor link to real languages that I can see in the general D&D rules of any edition.
4th Edition does include cipher-scripts¹ for several "languages" but doesn't actually present the languages, and I've not seen (nor looked for) examples of words in languages associated with them.
The partial exception to languages being described as limited in concepts is in the early editions... White Box, Holmes, Moldvay/Cook and Mentzer D&D, AD&D 1E. In these editions, mention is made of Alignment Tongues. These are limited to members of a given alignment, and as written, pretty much mean most races can find some language to communicate in. But, for the most part, alignment tongues are ignored in the published adventures, and generally also by a large portion of the gamers playing those editions. Moreover, there is only the vaguest implications that they might be limited in subject.
Now, there are some settings that include some terms in other langues. Most memorable is Dragonlance... Krynn's terms for the various elven races, and some placenames, provide some terms that can be pieced together, and one can analyze them for the sounds Elven in Krynn. Plus, there are lots of elven words in the very successful novels of that setting, so some con-langers² have been busy collecting them and attempting to rationalize them into functional languages.³
One can look through the Forgotten Realms and pull place-names, and extract phonemes from the various personal names as well, to establish the sounds, but there isn't nearly as much placed where its meanings can be grabbed as with Dragonlance. What I've seen are simple borrows from real languages and/or ciphertext⁴.
Similarly, the GAZ series has some hints at languages in various books, but nothing even as thorough as Dragonlance nor as much as in Forgotten Realms.
A related game, however, did make extensive use of a con-lang, back in the mid 70's... Empire of the Petal Throne. It was related to D&D in that it was a customized version of D&D, and sold by TSR, and remained mostly compatible. The setting and the Tsoylani con-lang are still going strong.
So, as far as adding restrictions on concepts, one could do so and not be outside the spirit of the game, but there's also not much done for you. Moreover, doing so may be beyond the scope play for many players, and seems to be beyond the scope for most groups.
The simplest way to make new language words, should you opt to do so, is to create a "fictionary" language⁵ - one that is simply english with made up words, and to generate a ciphertext or phonetic-cipher substitution table. I'll give you a two table example - one for vowels, and one for consonants. These are both a list which is then "rot 5"
Vowel - English: a e i o u y ow ar er or
vowel - cipher: y ow ar or a e i o u y
Consonant English 1: b c d f g h j k l m n p
consonant cipher 1: h j k l m n p q r s t v
Consonant English 2: q r s t v w x z ch ph sh th
consonant cipher 2: w x z ch ph sh th b c d f g
So, using the example, Watertown would become Shychuchit, and chit would be town, and Shychu would be water. Such cipher systems are readily used. Have fun with it. Note that one could also have other combinations on the table, change the rotation distances, or even assign the same output for 2 or more input letters, or have a randomization of 2 or more outputs for a single input.
¹: Cipher-scripts are scripts that simply replace extant letters with different looking letters, but provide a 1:1 or 1:2 correspondence with the extant character set. The 4E examples include a glyph for th, but are able to encode English.
²: con-langer - one who engages in the creation of constructed languages, or con-langs.
³: Elvish Lexicon at Dragonlance Nexus Wiki
⁴: Ciphertext - substitution by table lookup, often by numeric offset in alphabetical order. One of the most common modes of ciphertext is "rot13" - the lookup table is:
Rot13
Line 1: A B C D E F G H I J K L M
Line 2: N O P Q R S T U V W X Y Z
If the letter to be encoded is on line 1, replace it in the cipher with the one below it on line two; if it is on line 2, replace it with the one above it from line 1. It's called rot 13 because, if you place the letters in an alphabetical pair of wheels, starting lined up, the output wheel is rotated 13 places.
⁵: TVTropes.com Fictionary entry
