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As a side note: you are free to code in any version of JavaScript you want, but there is a user in this thread who thinks it should be ES6 compliant. Don't listen to him, because that would mean you will have to fix it to support SeaMonkey, as we have to regard the opinion of lambdatest.com and Mozilla more than anonymous user 'Verdy p' of translatewiki.net.
I am not anonymous. And I'm one of the oldest contributors of translatewiki.net, and have made lot of maitnenance work and support for helping many languages to startup or develop themselves, and added many missing documentation. But this site is not the support or developer site for any webbrowser. Translatewiki.net is an open project, and like all projects (free or not) it has limitations, most of tham based on available resources in humans and the time they can contribute, plus technical skills needed and sometimes money for the infrastructure. Most users here (including admins) are not paid for the work they already do for you and for all others since many years, as a best effort only, but without any contract with you. You are free to contribute, but if you can't, don't blame others if they can't help you more, when they explain their limits and propose you possible alternatives than most users accept without problem and that are available to you. If you don't like these alternatives, then there's nothing more we can do.
That's a fact that SeaMonkey does not support ES6 regexps (no support for non-BMP characters, it only supports "character classes" where characters are single UTF-16 code units, and treats non-BMP characters only as a pair separate "surrogate" code units, which count for 1 "character" in regexp character classes; as well no support for "\u", "\p{}" and "\P{}" and the "\U" flag, which are part of ES6): https://gitlab.com/seamonkey-project/seamonkey-2.53-mozilla/-/blob/2_53_14_final/js/src/irregexp/RegExpEngine.cpp
You should ask to SeaMonkey developers/support for such extension and revision of their regexp engine (note that SeaMoneky already has optional support for ICU, but it is not used by its regexp engine). Without it, transforming regexps is not evident: e.g. transforming ES6 regexps like /😀+/ or /[a😀]/ is not trivial: without ES6 support for unicode codepoints (not code units), these regexps will match more things than expected. The same affects regexps that would contain Chinese characters in the supplementary ideographic plane, or any attempt to match words written in Shavian, Deseret, and some modern scripts like Osage, Warang Citi, Adlam, Wancho and Toto, or existing MediaWiki translations for Gotic Wikipedia.
It's up to participants and supporters to the SeaMoney project to ask for ES6 compliance and help there if they can. But if they don't this is not surprising that this webbrowser is not even listed in https://caniuse.com/?search=es6 and that the browser has low usage. (non-ES6 browsers include IE11, and Opera Mini, bith of them also having limitations on this site, and IE11 is even no longer supported; Opera Mini remains only inside some old cheap smartTV's an a few appliances that should also not be used with Translatewiki.net). We don't really need all ES6 features in MediaWiki, but MediaWiki now depends on more and more components that need it (some support may still be present to emulatewhat is missing but it will be either slow or limited, and it is normal for these wikis to inform users with messages about things that do not work and that are not working, and will likely not work soon if this requires heavy development and tests, and constly maintenance that current developers have no way to support for long when they have many more things to do, fix and maintain elsewhere).
Side note 2: Mind the insane edit and re-edit history of this thread.