What does the one taste of the dharmadhatu really mean? What does the one taste of the dharmadhatu really mean? I know no language beside this English, and would be interested in any philosophical, literary, or religious answer on what "taste" means there, so I can best grasp what is being said.
In his commentary to Maitreya’s Mahayanottaratantra-ratnagotravibhanga, Gyaltsab Je says:


  Because just as every drop of water is of one taste with the ocean, so every phenomenon is of one taste within natural emptiness.


Geshe Jamphel Gyaltsen's explains:


  The absence of inherent existence of the mind of an ordinary sentient being, an arya or buddha is the same [in nature].


And:


  The final reality, the tathagata essence, the emptiness of inherent existence of the mind, is inseparable in the sense that all conventional phenomena are of one taste in emptiness. They are all inseparable in emptiness.


It does not mean that all phenomena are one and the same thing, for this would not be a Buddhist tenet. It means that phenomena are identical in that they lack inherent existence. Their lack of inherent existence can not be a basis for distinguishing them. In the same way, we say that sentient beings are identical in that they are subject to suffering while they want happiness. It is important to add “in this or that respect”, for we do not mean that they are one and the same objects.

A phenomenon and its final nature (its emptiness) are one entity. They are inseparable. Just like you can not separate heat from fire, you can not separate emptiness from its basis.

In a commentary to Maitreya's Distinction, it says:


  "All phenomena are of one taste in that they are without arising ultimately."



A second interpretation is that it means that, in the perspective of the wisdom directly realizing emptiness, no conventionality appear. A wisdom directly realizing emptiness does so in a non-dualistic manner. Since conventional phenomena are not experienced by that wisdom, we say that such a wisdom does not taste them.

This second interpretation is clearer in the visualization at the end of the Confession Prayer:


  Then reflect on emptiness. In emptiness there is no I, the creator of
  negative actions. In emptiness there is no creating of negative
  actions. In emptiness there are no negative actions created. Even
  though there are infinite phenomena, in emptiness nothing exists at
  all. There is no this and that, no me and you, nothing. In emptiness
  everything is one taste. From this emptiness, everything comes into
  existence. Whatever exists is the manifestation of emptiness.



By looking for "phenomena are of one taste", and "in emptiness, everything has one taste", you might find additional material.