But it's not true... You can replace vanilla strings with a modded value.
The issue was that if he does that (only for en-US.yml, other language files would not have side effects) then all other languages will need to be translated too (because en-US is always loaded on each level it is defined, regardless of UI language).
(Btw. this is a change that SupSuper did so that you don't have to copy en-UK.yml all the time...)
Example 1:
-----------
vanilla US: STR_DAY: "Day"
vanilla RU: STR_DAY: "день"
with this definition both vanilla TFTD and modded TFTD show correct translation
Example 2:
-----------
vanilla US: STR_DAY: "Day"
vanilla RU: STR_DAY: "день"
modded US: STR_DAY: "DAY"
modded RU not defined
with this definition:
vanilla TFTD shows "Day" for US and "день" for RU
modded TFTD shows "DAY" for US and also "DAY" for RU (instead of vanilla "день" for RU)
So when he tried doing this, russian players started complaining there are many things in US instead of RU...
... and yes, Nord could copy all vanilla translations into the mod translations to workaround... but that's ugly as hell... and when vanilla translations would change (which happens a lot for example in OXCE), he would have the old ones instead of new ones
PS: technically, the load order before was vanilla US, mod US, vanilla RU, mod RU... now the load order is vanilla US, vanilla RU, mod US, mod RU... both have advantages and disadvantages