When calling:

http://server/moodle/webservice/rest/server.php?wstoken=averylongstringforatoken&wsfunction=core_get_string&stringid=actionchoice

I get an invalid parameter error. In theory the rest of the possible parameters should use the defaults, the only required parameter according to the API is the stringid. The above value for string id has been taken from the lang/en/moodle.php file, the component defaulting to "moodle" and the lang defaulting to "en".

Discovered if you comment out line 90 in moodle/lib/external/externallib.php which is:

'lang' => new external_value(PARAM_LANG, 'lang', VALUE_DEFAULT, null),

Then this call works.