The ? stems from FactorioDataDeserializer line 568.

It arrives on line 565 with a luaTable containing a localised_descripption of 3 elements, an ?, and 2 other luaTable

Moving into the Localize(table) function, it decides to call Localize("?", table) that in turns calls FactorioLocalization.Localize("?") that returns null. Then on line 491 it calls table.ArrayElements(table), which returns just the "?". So basically it's just parsing what's in the data. I'm not sure how it should have parsed this different.

If i had to guess where this dataset comes from:

It seems pycoalprocessing_2.1.22\prototypes\functions\functions.lua:1302 adds a entity-description like this:

-- adds some text to a prototype's localised description
function overrides.add_to_description(type, prototype, localised_string)
	if prototype.localised_description and prototype.localised_description ~= '' then
		prototype.localised_description = {'', prototype.localised_description, '\n', localised_string}
	else
		if type == 'item' and prototype.place_result then
			for _, machine in pairs(data.raw) do
				machine = machine[prototype.place_result]
				if machine and machine.localised_description then
					prototype.localised_description = {
						'?',
						{'', machine.localised_description, '\n', localised_string},
						localised_string
					}
					return
				end
			end

			prototype.localised_description = {
				'?',
				{'', {'entity-description.' .. prototype.place_result}, '\n', localised_string},
				{'', {type .. '-description.' .. prototype.name}, '\n', localised_string},
				localised_string
			}
		else
			prototype.localised_description = {'?', {'', {type .. '-description.' .. prototype.name}, '\n', localised_string}, localised_string}
		end
	end
end

After going trough all this code, i'm still clueless what's going on here lol.

that's a new feature introduced in 1.1.73, officially called "fallback group"

for more information see https://forums.factorio.com/viewtopic.php?f=3&t=104293 and https://forums.factorio.com/71553

Thank you for the info!
Practically, do we want to hide this question mark, or do you have other ideas about how to handle it?

I was playing around to see if I could fix this. This snippet helps some cases e.g. Bonemeal (the item) is ? in main but with the following fix displays the right info. But it then causes items where there is no localisation (e.g. the diesel recipe) to show the localisation key: "recipe-description.diesel"

        private void Localize(object obj)
        {
            if (obj is LuaTable table)
            {
                if (!table.Get(1, out string key))
                    return;

                if (key == "?")
                    Localize(table[2]);
                else 
                    Localize(key, table);
            }
            else localeBuilder.Append(obj);
        }

What's the actual desired behaviour if a localisation is missing?

• Show ?
• Show the key (e.g. "recipe-description.diesel")
• Show nothing

If we follow what Factorio seems to do, it'd simply fail the first localisation lookup and return empty string for that specific item.
In this specific case ("recipe-description.diesel") that would mean there's no description for the recipe and the next part of the localisation occurs (the "affected by productivity" string is put in the description).

#63 might be useful as a starting point for fixing this properly.

What's the actual desired behaviour if a localisation is missing?

I think it's either to show nothing or the key. There are arguments for both of the approaches, so it's up to you.

Wouldn't you default to english then? If that's also missing i guess the above.

Wouldn't you default to english then?

From what I see, YAFC already uses English everywhere, no matter what language your system or the game is using.

Not for me, it has the same parts dutch as Factorio has ingame.

The in-game items are localized (by the mods), but the whole application itself uses (hard-coded) English. Open some files and you see all kind of strings in the code to be rendered in the UI. (Unles SDL applies some magic to match them against localisation files to translate?)

Here are the before and after for what's currently implemented in #63
It tries to match what's what Factorio does as closely as possible.

Before:

After: