A pledge to make your language free (as in freedom defined by FSF [1], so its antonym is proprietary) is not an open source by itself. Currently your language is not free.
EDIT: s/open source/free/
EDIT: s/open source/free/
You can't call an unreleased language proprietary.
Many current programmers think F/OSS as a norm and proprietary (released or not) as exceptionally shady. Even though some may think proprietary software is acceptable, something being F/OSS is a label with the non-trivial amount of value. Your act can be interpreted as claiming that value without actually being F/OSS, no matter what was your intention.
Without releasing it you're expecting people to put blind faith in you.