For the duration JustOff was the head of the official Pale Moon localisation project, he would maintain his own langpack (LP) GitHub repository,
https://github.com/JustOff/pale-moon-localization/releases
and his Pre-release LPs would target PM's unstable channel, with which NM28 has/had the closest affinity...
Modifying the minVersion value inside the install.rdf file, to account for NM28's different versioning, would produce an installable and functioning langpack...
This practice used to work up to a point in time (e.g. I know it worked till the end of May 2019, when NM28 was at v.28.6.0a1); but,"upstream" started making coding decisions that were deemed "unwanted" by the majority of members here; old code was kept by us, but removed by upstream (e.g. WebIDE inside devtools) and vice-versa, i.e. they introduced new code that was not adopted here...
Whenever these code differences introduce string modifications (addition or removal), then the PM-targeting LP becomes incompatible with NM28 to a varying degree; slight modifications in parts of the browser you don't use often may stay unnoticed, but when major core strings have been altered, considerable breakage takes place, even to the point that the non en-US pack renders NM28 unable to launch with it; a "safety" mechanism then silently disables that LP and re-launches the browser in the default (en-US) locale...
I wrote the above "summary" for the sake of those unfamiliar with NM's localisation scenarios; FWIW, roytam1, from the very start, said he would not provide himself LPs for his browsers, we, as fork users, were just lucky we could, up to a point, "freeload" the ones prepared for PM by MCP (and get all the hatred from them for doing so... ).
Our (roytam1's ) source tree had already diverged significantly from upstream even before JustOff was ousted from the MCP localisation team, but lack of his "unstable" LPs currently means that a DIY person like you can't peek into them to find newly introduced strings there (that could be migrated to a NM28-specific LP); the sole source of PM localisation now are the LPs intended for its release channel, to be found:
https://addons.palemoon.org/language-packs/
with NL on https://addons.palemoon.org/releases/pm-langpack-nl/
Save as .xpi file and extract with 7-zip, then look inside for new strings not yet incorporated into your own NM28 NL-pack; you have to be well versed in the folder/file structure of the LP, if you have a NM28 NL pack that used to work in previous NM versions, you can "build" on top of that and go forward towards more recent releases...
You must use a file diff application (or a text editor) to inspect the string differences between the exact same files of the two LPs, so you can modify your DIY one accordingly (taking care to incorporate only the new strings needed by our fork, the PM file may contain other strings not compatible with NM).
Indication of strings modification in code can be found also on roytam1's GitHub repo; whenever UXP and/or Pale Moon source files that have a path containing "*/locales/en-US/*" are being modified, this signals modification to the natively bundled en-US langpack; these modifications should be reflected (and translated into Dutch, where applicable) in your DIY NM28 langpack.
For instance, for the "Media Formats" strings that you cited, the relevant commit is
[Pale-Moon] Issue #1862 - Add media format controls in Preferences -> Content
from which you can see the newly introduced (English) strings in file
application/palemoon/locales/en-US/chrome/browser/preferences/content.dtd
by loading:
https://github.com/roytam1/UXP/commit/650abe2cb9eb02c14369a807a6225ebb053d0b17#diff-d3c90e964941daf05f84a5b5192bc0c94f6f629dcc424dd23e8f87cc766cd4f7
Many times, it is difficult to tell on which LP file to apply the string changes committed to source files, this ain't one of them :
<LP root>\browser\chrome\nl\locale\browser\preferences\content.dtd
In conclusion, the natively bundled en-US langpack is found split between NM28's two omni.ja files:
".\palemoon\browser\omni.ja\chrome\en-US\*"
".\palemoon\omni.ja\chrome\en-US\*"
(use 7-zip to access); ultimately, these are all the files you have to translate into Dutch and then package appropriately (consult an existing LP) to make a functional NM28 nl-langpack.xpi; it ain't easy, is it? But I am open for any automation process you happen to be aware of (this extends to the rest of this community here ) ...
Kind regards