It's moments like this, when I'm thinking along these lines, that I realize just HOW MUCH of a geek I truly am. X_x;;
The Tayledras language is a very indepth language, if incomplete. The reason for its lack of completion, and its limited uses and meaning, is quite simply because the inventor of this language only NEEDED to get certain points across in the Tayledras tongue. For the most part, those who spoke Tayledras rarely intermingled with others, and those who DID often spoke the common language, which was roughly classified as English, since that's the language the text was written in.
Ergo, while the language has a definite depth to it, it lacks broad meaning. It can only truly be used, for the most part, to emphasize the precise depths of interpersonal relationships, especially as three of the four most common terms in the language, or in the known language (ke'chara, shayana, and shay'kreth'ashke) all refer to specific interpersonal relationships and their specific value with the speaker. (Shay'a'chern, being the fourth common term, relates not to the speaker's interpersonal relationships, but rather the gender of the indicated individual's romantic/sexual relationships, as shay'a'chern translates to "one who loves like self", with a gender-based connotation on self rather than a familial one.)
However, using the known connotations of various prefixes and suffixes, it is entirely possible to create new, and even in some cases more powerful, meanings than the commonly used ones. For instance, "ke'chara" is a generic phrase, which literally translates to "dear teacher", and is used to reference most close friends with whom the speaker is not intimate. However, a simple added prefix gives the word "kesh'chara", which added a closer intimacy, if not necessarily a physical one, and adding another root would give "kesh'kreth'chara", giving the relationship a definitive power, as "'kreth" is considered a lack of temporal ending, or more specifically, translates to "lifelong". (As in "shay'kreth'ashke", which translates roughly to beloved soulmate, or specifically to "lifelong love of self", but with a differential to separate the other from self, making a clear depiction of a soulmate.)
So, from "ke'chara", we get "kesh'kreth'chara". The emotional and physical bonds of the friendship are clearly deepened, but what of spiritual? Not to mention, these words could get a bit cumbersome to speak, for while a single word has all the meaning of an entire sentence, and then some, it still doesn't roll off the tongue as easily as one might want. I think, perhaps, the inventor of this language kept the language to basic minimums as much as possible for this language, as some ideas would be cumbersome to express in this language, both depsite AND because of the entire thought being compressed into a single, heavily multisyllabic word. Perhaps she intended, at some point or other, to impose a bit of shorthand upon the language, so to speak. Find a way to condense certain combinations of ideas into a more easy-to-speak format, similar to the slang "shaych" in reference to "shay'a'chern". This wouldn't surprise me, especially as, when multiple languages combine and/or conflict, slang and shorthand both are commonly the first to appear, to help ease the linguistic barriers.
But, what shorthand could we concoct for the term "kesh'kreth'chara"? The ideal speaks of something just barely below "shay'kreth'ashke", but separated sufficiently to make it impossible to mistake the two... while the first insists of physical and emotional closeness, and likely mental as well, the second is that of bonded souls, of soulmates. There's little comparison between the two, honestly. Too, the term "shayana" (defined as wingsibling, which is a closeness of mind and spirit, and sometimes of lineage, although there have been "adopted" wingsiblings before, Savil and Vanyel being two such cases) is too simple a term, as it does not exemplify the precise level of closeness, as "kesh'kreth'chara" does.
Also, why "'chara" and not "'ashke"? Some might think they would be interchangable, and in this precise case, it's entirely possible. The definition would change from a lifelong friend, who is physically and emotionally attached to the speaker, to a lover, similarly attached but not necessarily lifebonded. The only exception here is a slight differentiation in the level of "physical closeness", to wit. But, in this day and age, that is not truly so uncommon as to be ludicrous.
Perhaps I'm trying too hard to read into the language. I'll let someone else consider that. However, I think the term "kesh'kreth'ashke" is about the perfect descriptor I can come up with. A lifelong love, with a level of closeness emotionally and mentally which is akin almost to siblings, and a level of closeness which allows them to be termed lovers as well as loves.