I discovered this issue as well.
The problem is a mismatch between the external function parameters returned by get_string_parameters() and the actual method signature of get_string: get_string does not have a lang parameter. However the definition below specifies one (see below).
public static function get_string_parameters() {
return new external_function_parameters(
array('stringid' => new external_value(PARAM_STRINGID, 'string identifier'),
'component' => new external_value(PARAM_COMPONENT,'component', VALUE_DEFAULT, 'moodle'),
'lang' => new external_value(PARAM_LANG, 'lang', VALUE_DEFAULT, null),
'stringparams' => new external_multiple_structure (
new external_single_structure(array(
'name' => new external_value(PARAM_ALPHANUMEXT, 'param name
- if the string expect only one $a parameter then don\'t send this field, just send the value.', VALUE_OPTIONAL),
'value' => new external_value(PARAM_TEXT,'param value'))),
'the definition of a string param (i.e. {$a->name})', VALUE_DEFAULT, array()
)
)
);
}
I will do as Bill suggested, ie comment out the 'lang' parameter definition from the get_string web service call. However that only addresses this issue temporarily ( and only for me .. and Bill 
).
Has this been resolved in a later version of Moodle? I too am using version 2.6.
I'm not a PHP developer (I spend most of my time in java) and I'm more used to stronger typed languages and so not at all used to this type of scenario. Nonetheless I have a suggestion that could help guard against this parameter/mismatch and create a more tightly checked system.
Is it possible to write a unit (or integration) test that iterates through all the externally defined functions (SELECT * FROM mdl_external_functions) and validates that all the function parameter definitions actually match up with the function signatures. Not being a PHP dev I'm not sure if you can extract that sort of meta data. Anyhow, just a thought. No idea if this is doable.
