FTR: https://forum.palemoon.org/viewtopic.php?p=185808#p185808
@Alex654 and others feeling the urge to do that: PLEASE, NEVER TAKE ISSUES SPECIFIC TO NEW MOON/SERPENT to the official Moonchild forums! What you'll only achieve is to cause additional aggravation to the devs (especially to a certain Mr. Tobin) who, as most people here know already, harbour animosity towards the forks for "old" OSes...
The thing is, I have fought hard in these threads in the past so that the New Moon 28 fork doesn't deviate significantly from the official Pale Moon code with respect to language strings, in order to keep parity with the official PM language packs which, I stress, are only being produced to target (and be fully compatible with) official Pale Moon!
The fact that we were able to install and use the official LPs (to be more precise, the ones compatible with NM28 were the ones offered by @JustOff to target the official unstable Pale Moon builds) was just a side bonus, these packs do contain the official branding and I'm sure the MCP devs weren't/aren't happy we were using them to begin with...
Due to certain coding decisions taken by our maintainer ( @roytam1 ), the custom UXP fork branch he's compiling NM28 off has distanced itself from upstream (which already uses a different build system, i.e. official unstable PM29 is now being compiled from a dedicated PM repo, with official UXP as a module), so that the LPs currently produced by @JustOff are simply not compatible with New Moon; from past discussions, it is my belief that we are not to see any updated LPs for New Moon coming from @roytam1 himself, so I guess it's a task to be undertaken by the user community...