For new OA publications at conferences/journals, authors get a choice of CC licenses [1,2]. The respective licenses can be found in an article's [3] "Information" sidebar (hidden by default) or the HTML/PDF version [4]. Unfortunately, the licenses are apparently not shown more prominently in the ACM Digital Library. The search function does not allow filtering by license either.
For CACM articles, the traditional ACM copyright notice seems to apply [5] (maybe there are CC-licensed articles, but I did not see any so far):
"Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for third-party components of this work must be honored. For all other uses, contact the Owner/Author."
For pre-OA publications (< ~2020 ?), authors could either transfer copyright or give ACM a publishing license [3]. I'd assume that ACM may assign an arbitrary CC license to publications to which they own the copyright but not to ones for which they only have publishing rights. Given that authors only licensed publishing rights for the final article to ACM, I'd guess the only acceptable CC license in this case would be CC-BY-ND.
However, I guess that ACM won't want to go through the trouble of assigning CC licenses to older publications.
EDIT:
My tentative guesses/answers to your questions would be:
> - how effectively libraries' rights to archive, copy, and redistribute works published in cacm is protected;
IIRC, libraries subscribing to ACM's DL already have the right to archive the content. Given that many authors have only given ACM the right to distribute their works, I'm not sure whether scraping OA articles is allowed. (ACM has been banning IP ranges for mass-downloading papers in the past, so be careful.)
> - whether the license is compatible with incorporation of text into wikipedia and stack overflow; and
IMHO, only if a non-ND CC license was applied to the article. Also, correct attribution is still needed [7].
> - to what extent volunteer translations into languages such as chinese and spanish can be legally archived, copied, and redistributed
Again, probably only for explicitly CC-licensed content (excluding -ND).
[1] https://www.acm.org/publications/policies/publication-rights...
[2] https://cfiesler.medium.com/acm-copyright-licenses-which-sho...
[3] https://dl.acm.org/doi/10.1145/3623509.3633366
[4] https://dl.acm.org/doi/fullHtml/10.1145/3623509.3633366
[5] https://dl.acm.org/doi/10.1145/3624717 (sidebar)
[6] https://academia.stackexchange.com/questions/14734/which-acm...
[7] https://en.wikipedia.org/wiki/Template:Creative_Commons_text...