Since the break-up of Yugoslavia, the Sava river, which was the largest national river in Yugoslavia, became an international river, passing though Slovenia, Croatia, B&H and Serbia (at the time of still the FRY). To regulate the international navigation and to ensure the protection of the environment, the International Sava River Basin Commission (ISRBC) was established in 2003. Since 2005 the seat of this international organization is in Zagreb, following the agreement between Croatia and the organization, thus getting diplomatic status usual for such subject of international law. The organization is based on following the similar organization on Danube, and taking into account of its heritage (since the DC "responsibility" is on the whole Danube basin, so also includes Sava).
Although the cachement of Sava river and thus the river basin reaches partially into Montenegro and with small area in Albania in the south and also very minorly in Austria in the north, these parts are not navigable and thus apparently not members of the organization.
It seems that Serbia only inherited the agreement rights and obligations from FRY, and not Montenegro.
the Commission is established by the Article 15 of the document, fully named International Sava River Basin Commission, and with the short name Sava Commission.
The Article 8 of the Statute of ISRBC which is Annex I of the Framework agreement, the official languages of its bodies are determined to be "the official languages of Bosnia and Herzegovina (Bosnian, Croatian, Serbian), Croatian, Serbian and Slovenian" while "The Sava Commission may use other languages if necessary."
The Framework Agreement in the official languages may also be found on the web site, where the names are stated so:
Slovenian: Mednarodna komisija za Savski bazen (Savska komisija)
Croatian: Meðunarodna komisija za sliv rijeke Save (Savska komisija)
B&H (Bosnian): Meðunarodna komisija za sliv rijeke Save (Savska komisija)
B&H (Serbian): Међународна комисија за слив ријеке Саве (Савска комисија)
B&H (Croatian): Meðunarodna komisija za sliv rijeke Save (Savska komisija)
Serbian: Међународна комисија за слив реке Саве (Савска комисија)
The emblem of the organization is a wavy line crossing four squares along its path, two behind and two infront of it
I was unable of finding any mention or reference to flag of the organization
Flag, Seal, Emblem and Logo
1) The Sava Commission shall have its flag, seal, emblem and logo.
2) The Sava Commission shall adopt a decision on design and use of its flag, seal, emblem and logo.
Somehow I missed this (trusting the search engine on the site, instead of using google...)
The ROP is available on line in all the official languages (except I haven't found the Slovenian one), where in each language the flag is translated, of course, as "zastava" (Serbian using Cyrillic script). This is not unusual at all, but just for the record.
Now I found some visual proof of its flag. Image of flag (bottom of page 4 in PDF), and desk flag version.
I believe (but the photos really do not show it clearly) that the text is the commission name, probably English under the logo and Croatian above (since the seat of the Commission is in Zagreb). However, for the big flag in the PDF you sent link of, I am not quite sure if it really does include the inscriptions - maybe it is just the logo without text...