Factorio Codex version 0.0.18 is not compatible with Factorio library / flib 0.12.6 (Factorio Version 1.1.76).

Upon starting the savegame and pressing 'n' to search, the "Waiting for translation" error still pops up.

This seems to be a programming error in Factorio Codex as flib matches.

"
Error while running event factorio-codex::on_gui_selection_state_changed (ID 60)
factorio-codex/build/QuickSearch.lua:299: attempt to index local 'selectedResult' (a nil value)
stack traceback:
factorio-codex/build/QuickSearch.lua:299: in function 'gui_action'
factorio-codex/build/events.lua:73: in function <factorio-codex/build/events.lua:67>
"

I am not completely sure what your problem is so I will tell you what I understand and you can correct me and show me what you actually meant.

Looking at what you wrote I understand it like this:
* The search result "Waiting for translation" only shows up when there is an error.
This is not the case since it only shows the status of the translation request. When any mod gets added/updated/removed translation has to be done again and in this time you can see the message "Waiting for translation".

The title of this thread suggests that you mean:
* The translation does not progress at all and I am unable to use this mod.
In this case you would see a green progress bar in the top left of your screen that is stuck at a low percentage like 0% or 1%.
This used to happen with flib < 0.12 and factorio 1.174 since the factorio devs slightly changed the way how translation works and the old flib versions are unable to process the new way correctly.

I also took a look at the error message you provided but this seems to be a different bug which I'll fix of course.
This bug can be avoided by not clicking the "Waiting for translation" item in the search results.
(The mod thinks everything in there is either a result from a math formula or an item which it can show in the codex. Since its not a number it tries to open the codex and shot that "item". This does not work since its obviously just a message for the user to be a bit patient and then the mod crashes)

Regardless of what you mean I tried to reproduce your issue with a new save and the provided version information but this mod worked as intended and the translation completed successfully.
To narrow this issue down and find the actual reason I need the following information fro you:
* Did you have this mod installed before the last update? (If yes which version?)
* Does this issue also occur for you on a new map?
* Can you provide the factorio log file of this crash? (You can find the log file in "%appdata%\Factorio" with the name "factorio-current.log" or "factorio-previous.log" if you're under Windows, or just take a look at the wiki if you're not on windows https://wiki.factorio.com/Application_directory#User_data_directory)