I imagine most voters do not read proposed bills in their own language as it stands, instead relying on reporters' summaries as they would for foreign language versions.

Agreed, but that's not a reason to write laws or treaties in a language the general population does not speak. That's too high a barrier to understand the rules of your nation, which should be easily accessible to every citizen. Legalese is complicated enough in your native tongue :).

AIUI, a treaty is not law unto itself. Rather, the parties to a treaty will pass laws that implement said treaty.

This is not relevant for the political side of the question, e.g. it would not be possible for a citizen on its own to judge whether the merits of a treaty. Say, your senator campaigns on withdrawing from the Iran deal: how are you supposed to assess this position if the deal was in third-party-language Pashto?

In addition, I can present the case of France, where judges must disregard laws that contradict treaties (the hierarchy of norms being Constitution > Treaties > Laws > Executive orders). Therefore treaties, even with no national law to implement them, have a direct effects on the law.

Treaties are accorded to the power of law in the treaty signatories, below their Constitutional documents but above that of legislative laws.

Once a treaty enters into effect, a nation is bound by the terms of the treaty even if they do not have implementing laws. However, in many cases treaties don't have any consequences if signatories don't pass implementing laws (see, e.g, the Kyoto Protocol).