Document possible values for --lang="LANG"

I am following [the installation instructions](https://docs.civicrm.org/installation/en/latest/general/cli-cv/) for Civi and I'm stumped at how to build the command to install civi. I am a complete Civi Novice, but I think it will be exactly the right solutuon for one of my clients, so I really want to give it a try. The example provided is `cv core:install --cms-base-url="URL" --lang="LANG"` But there are things in here I don't know how to replace -- let's start with `LANG`.

I can tell this is language, and I can see that there are notes below stating that if I want to install a non-standard language, I need to have the language files downloaded, and the codes need to match. However, none of that is relevant for me, I want to install Civi in the default language.

What code do I put in here for the default language? Do I need to specify the code for US English, and if so, what would that be? Is it the lowercase `und` like it is in Drupal + Backdrop when the language is undefined? Or is there a constant to be used like `LANGUAGE_NONE`? Is it an ISO language code? (but `fr_CA` does not look like an ISO language code...) Is there a list of possible values? Is the example provided `fr_CA` the most commonly used language, and therefore probably US English? (My gut says no, this is probably French as used in Canada, but that doesn't help me determine what US English might be.)

My gut also said that maybe I should leave off the `--lang="LANG"` parameter entirely if I want to install Civi in the default language. But if that's the case, why would it be included in the "typical installation command" example?

(the above was my complete thought process while trying to install civi -- maybe not the best format for this ticket, but I wanted to write it all down while it's fresh in my mind.)

Things that might help me determine what to use for LANG:
* An example command of how to install Civi in US English (probably the most common use-case?)
* A complete list of allowed language values, including the default value
* An indication as to whether the `--lang` parameter is optional or required

Also, if there is documentation I should be consulting other than https://docs.civicrm.org/installation please let me know if I am looking in the wrong place :)

Start with the installation instructions at docs.civicrm.org/installation/en/latest/general/cli-cv/ and inspect the cv core:install command's --lang behavior. Clarify the default language, accepted values, and whether the option is optional, then update the typical installation command and surrounding documentation with a US English example and a complete value list.

Written by the indexing model from the issue text.