We add a prefix to id of default_language_modal.hbs
such that we can use the same code for user settings
and realm-level settings.
We also add a class "default_language_modal" to the
modal div to avoid duplicate css.
The language_list_dbl_col parameter in the page_params
is used by only the web client frontend. The value is
calculated in the backend and then passed as a page_param
which is unnecessary considering that the whole process
is beneficial for the front_end only. Hence move the entire
calculation code to the frontend.
Fixes part of #18673.