Exported game output this into terminal:

WARNING: Translation remap 'res://font_en.tres' does not exist. Falling back to 'res://font_en.tres'.
     at: _path_remap (core/io/resource_loader.cpp:1197)
WARNING: Translation remap 'res://font_ja.tres' does not exist. Falling back to 'res://font_en.tres'.
     at: _path_remap (core/io/resource_loader.cpp:1197)

Seems that this 2 resources doesn't get included in exported build?

Exported game output this into terminal:

WARNING: Translation remap 'res://font_en.tres' does not exist. Falling back to 'res://font_en.tres'.
     at: _path_remap (core/io/resource_loader.cpp:1197)
WARNING: Translation remap 'res://font_ja.tres' does not exist. Falling back to 'res://font_en.tres'.
     at: _path_remap (core/io/resource_loader.cpp:1197)

Seems that this 2 resources doesn't get included in exported build?

Yes, I tried exporting them explicitly, but it still didn’t work.

Adding new labels and applying that themes to them - works, so resources definitely included in exported build.

Adding new labels and applying that themes to them - works, so resources definitely included in exported build.

My tests reached the same conclusion. It seems that the remaps resources are not shared with the regular resources, or not properly linked for some reason.

I'm having this same issue and error message, but on a custom resource, unrelated to themes, and only in Web exports.
I tried even using metadata to reference a resource that points to all locale variations, but no luck.
I guess I'll have to avoid the localization remap system entirely and use my own code for this... 😞

I am facing the same issue, I think it's related to #84791
After checking my .pck file, I noticed that the extensions of remaps is .remap

For example, my file main-menu-theme.tres will become main-menu-theme.tres.remap rather than main-menu-theme.tres.import
So how to fix this? I think we need to skip also the extension .remap

I will see if I can test this, my project uses mono, it's much harder to compile, but I will try

	if (!FileAccess::exists(new_path + ".import") && !FileAccess::exists(new_path + ".remap") && !FileAccess::exists(new_path)) {
			WARN_PRINT(vformat("Translation remap '%s' does not exist. Falling back to '%s'.", new_path, p_path));
			new_path = p_path;
		}

I can confirm that the inclusion of ".remap" in file exists solves the problem
Will create a PR for this

Created the PR #103838
You can test it if you would like to, but a export template will have to be built for each target. Which is a lot of effort.