How to find tcl procedures that do what you want: go to /api-doc/ of your local installation, type in a meaningful search term, e.g. 'parameter', and go through the results. In this particular case you have been looking for the proc parameter::set_value. (
https://openacs.org/api-doc/proc-view?proc=parameter%3a%3aset%5fvalue )
Note that procedures with :: in it are generally preferred because they use the more modern namespace feature and are propably newer.
An addition to the default-master hint above: I noticed that it won't work for pages under www/, unless you specify a full path like this:
ad_return_template "/www/default-master-$language"
