I am playing Krastorio 2 + Space Exploration at the moment and I get unknown key errors for:

entity-description.kr-steel-pipe
entity-description.kr-steel-pipe-to-ground
entity-description.spitter-spawner

I found even more. I start to get more suspicious about Space Exploration. I think Earendel is messing things up a bit when adding the warnings about "cannot be placed on space platform" or something -_-

It is impossible to tell if an entity has a localised description from inside the code, so I either need to ignore all existing descriptions, or try to include the description if there is one but have the "unknown key" text for the ones that don't. It's obvious what the correct choice is.

The only fix is for one of the mods to include descriptions for the entities that are affected this way, even if the description is just an empty string. it doesn't matter which mod adds the description. You can help by listing the affected entity names, I can add them to the postprocess mod if Linver doesn't want to add them to K2.

It is impossible to tell if an entity has a localised description from inside the code

This sounds like an obvious problem with the LUA API. Maybe a strong case for an API request?

But just to clarify: I am playing in English at the moment. This is not an issue about translations.

The two vanilla spawners don't have a description, which is entirely unrelated from Krastorio 2.
entity-description.spitter-spawner
entity-description.biter-spawner

Specific to AAI Vehicles, most likely also unrelated do either Space Exploration or Krastorio 2:
item-name.flame-tumbler-flamethrower

Specific to Krastorio 2:
entity-description.kr-filtration-plant
entity-description.kr-electrolysis-plant
entity-description.kr-steel-pipe
entity-description.kr-steel-pipe-to-ground
entity-description.kr-atmospheric-condenser
entity-description.kr-bio-lab
entity-description.kr-advanced-transport-belt
entity-description.kr-advanced-underground-belt
entity-description.kr-superior-transport-belt
entity-description.kr-superior-underground-belt
entity-description.kr-advanced-chemical-plant
entity-description.kr-advanced-furnace
entity-description.kr-advanced-assembling-machine
entity-description.kr-matter-plant
entity-description.kr-stabilizer-charging-station

Entities that do not show the "Cannot be placed on: Space platform, Spaceship" warning although their cousins do:
entity-description.kr-advanced-splitter
entity-description.kr-superior-splitter

Ok, I can add this descriptions, but should they be empty strings? Or they can be full/right for the related object?

An empty string is fine, but to make use of the line reserved a 1-line string related to the object is more helpful for the player.

Later I will push the changes. Me and Krastor plan to do an update on 07/11

Done:
https://bitbucket.org/Linver23/krastorio-2/commits/bccba12476c76287648062608ee61f6fffedc281