I agree with Ron here.
Language tutoring is more equivilant to piano lessons, or guitar
lessons, or drum lessons.
It is a skill which takes a long time to master, but
can be acquired in short amounts, even to the point
of having complete small conversations (just
like a piano learner or guitar student will be able
to play a simple song).
It gets more complex over time, and the learner's
knowledge is more complete about this one particular
language (whether the language be a musical instrument,
or the vocal chords in foreign language speech), and in
both cases, interpretation takes place in the brain.
So, whatever piano teachers or drum instructors are paid
for tutoring, base your fee around that.