Frame Challenge About Simple Languages
Some of your assumptions about "simple" languages are not as simple as you may think.
A simple language can have subjects, verbs and articles in any order and the phrase still make sense.
A simple language should not force information into grammar, in many languages one can not speak without specifying time, gender and sometimes location or quantity. The amount of information to be shared with each phrase has to be optional and not enforced by grammatical rules.
Languages with looser grammar rules require more clarifying language to make since. So lets pretend you tried to say "He has a daughter with a yellow hat" in such a language, it could be said:
- With he daughter has yellow hat
- Daughter has with he hat yellow
- Has yellow hat he with daughter
- He yellow daughter hat with has
As you can see, there is no way to communicate what the words mean together with grammar thrown out the window. The only way to disambiguate the language would be to make it more complex by adding a bunch of prepositions or conjugations to tell you how the words relate. So to have clear meaning the language would then look like:
- Proposition-hat-with subject-he object-daughter verb-has adjective-hat-yellow object-hat
- Object-daughter verb-has proposition-hat-with subject-he object-hat adjective-hat-yellow
- Verb-has adjective-hat-yellow object-hat subject-he proposition-hat-with object-daughter
- Subject-he adjective-hat-yellow object-daughter object-hat proposition-hat-with verb-has
Now you have made the sentence able to be clear without most of the time without grammar rules, but in the process, you've made the language very unnecessarily complex. Grammar in most cases simplifies languages by giving you a structure on which to make common assumptions about the relationships between words.
Grammarless language is also unpreservable because it will naturally start to form thanks to common biases. Eventually groups will settle into word orders based purely on how other people choose to order their words.
Instead what you want is actually a very strict Grammar, one without exception or irregular conjugations. So, if a noun can end in a conjugation that implies gender (like: male, female, and unspecified) it should ALWAY NO MATTER WHAT end in a gender conjugation, then you can get rid of all those pesky unmatched gender specific words like butler/maid/housekeeper. Instead you are a maido, maida, or maidum: 1 word that can be predicted and fit according to a common rule. Or you can treat gender specification as a separate and optional word that is always appended in a fixed position relative to a noun so you could have a "he maid", a "she maid", or an "it maid". The reason you don't want optional NULL conjugation is so that you don't get concatenation collisions. For example let's say you have two base words "med-" and "medum-". By making Null conjugation optional, "medum" could mean either "med-" with the "um" sufix or "medum-" unspecified. But if "medumum" was the only proper conjugation for "medum-" then you are less prone to language collisions.
As for Maintaining a Language
Historically this has proven very elusive because most people learn to speak from non-canonical sources, but today we have the ability to establish canonical language standards much better. Especially if you were to replace English teachers who each have thier own biases with a standardized language teaching AI. To freeze a language, you need an official language guide that is taught as a part of your culture's compulsory education system. When you first introduce it, you need a guide that tells you what the canonical rules and words are. A good modern example of this is Octopuses an Hippopotamuses. For a long time, these English words retained their Latin plural conjugations of Octopi and Hippopotami, but the public schools started teaching English regular conjugation of these words and within a few decades, the English regular conjugation has become the preferred method.
Basically you do this, but on a MUCH grander scale. You take the language everyone knows and you take a hack-saw to in in public schools. If a student writes "avion" instead of "bird" on his biology paper, that's -1 point. If he writes butler instead of maido, that is -1 point. Basically, you do what we are already doing in schools to homogenize language, but with the added directive to also simply it.
In this process you can also weed out same or similar sounding words by choosing cannon words based partly on their uniqueness. "Their", "there", and "they're" would be unacceptable. So, "there" could be canonized to "that place" and "they're" to "they are".
Lastly, you write down all the rules and make sure make sure the governing body responsible for language is tasked with the mission of preserving language rather than adapting to it. The problem with things like Merriam-Webster or Oxford dictionaries is that it is their goal to reflect the current meanings of words; so, they require panels of experts to constantly add & subtract words. When I was little, there was a common saying that "ain't ain't a word 'cus it ain't in the dictionary", yet today ain't can be found in many dictionaries because of its common usage. Instead whoever is in charge of your nation's dictionary on which children are taught the one-and-only acceptable method of speaking, it is their goal that when a new word starts to arises, that they will try to fit it to canonical word. So when people start using new words things like "Chillax'en" or "cellphone" the public board of language will examine the word to decide if it is infact a new thing or if it needs to be canonized to a proper word. So "cellphone" might get to become a new word because it did not exist before, but "Chillax'en" would be officially canonized to "resting" along with "chilling", "relaxing", "hanging out", "loitering", "idling" and any other word that means to spend awake time doing nothing of interest.
Under this system, colloquialisms will of course still pop-up because it is human nature, but they will also die out reverting back to the original language with each generation that passes.