The simple answer is that legal language like this is often formulaic, in that it follows certain prescribed patterns, which are often very different from how English is commonly spoken. It is often formal, repetitive, dense and often hard to understand, especially for people who are not familiar with the language.
In this case the edict has some redundancy to make sure that there is no room for misinterpretation; however the "do establish" implies more the English quirk of using the present tense to indicate future events. "We do establish" means "we establish for now and the foreseeable future" (that some thing is true).
Also as Tᴚoɯɐuo mentions, the original was written in French, not English, so this is perhaps an accommodation to mirror in English what could be written in French as a single phrase, or to match certain similar formulaic legal patterns in French. It may sound strange in English, but there is no real harm in being thorough.
As another example, the preamble to the North American Free Trade agreement, signed in 1993:
The Government of Canada, the Government of the United Mexican States and the Government of the United States of America, resolved to:
STRENGTHEN the special bonds of friendship and cooperation among their nations;
CONTRIBUTE to the harmonious development and expansion of world trade and provide a catalyst to broader international cooperation;
CREATE an expanded and secure market for the goods and services produced in their territories;
etc.
All present tense verbs in a past context ("the three nations resolved"), although the present perfect would have been fine ("the three nations have resolved").
(Edit). Whoops. As tenebris2020 points out, these are all tenseless infinitives, not present tense verbs. So let's look further into the text of the treaty:
The Parties to this Agreement, consistent with Article XXIV of the General Agreement on Tariffs and Trade, hereby establish a free trade area.
The Parties affirm their existing rights and obligations with respect to each other under the General Agreement on Tariffs and Trade and other agreements to which such Parties are party.
However, much of the document is written using "shall" to form the future tense, with the additional nuance of obligation. I expect the use of one or the other has subtle legal connotations, but I'm not experienced enough with this kind of language to know what those are.
