fix(locale): deep-merge fallback into partial namespace returns

svelte-i18n's getJSON returns the active locale's sub-tree verbatim;
nested keys missing in that locale do not auto-fall-back to the
fallbackLocale. The locale store now deep-merges the English value
into the resolved object so JP users see English for any sub-key the
JA dictionary omits (instead of blanks).

1 files changed, 38 insertions, 7 deletions

diff --git a/src/stores/locale.ts b/src/stores/locale.ts
index 826d717c..1b94ea96 100644
--- a/src/stores/locale.ts
+++ b/src/stores/locale.ts
@@ -25,6 +25,22 @@ interface Options {
 	values?: InterpolationValues;
 }
 
+const FALLBACK_LOCALE = "en";
+
+const isPlainObject = (value: unknown): value is Record<string, unknown> =>
+	typeof value === "object" && value !== null && !Array.isArray(value);
+
+const deepMerge = (fallback: unknown, current: unknown): unknown => {
+	if (current === undefined || current === null) return fallback;
+	if (!isPlainObject(current) || !isPlainObject(fallback)) return current;
+
+	const merged: Record<string, unknown> = { ...fallback };
+	for (const [key, value] of Object.entries(current)) {
+		merged[key] = deepMerge(fallback[key], value);
+	}
+	return merged;
+};
+
 const createLocale = () => {
 	return derived(json, ($json) => {
 		return (options: Options = {}) =>
@@ -32,9 +48,24 @@ const createLocale = () => {
 				{},
 				{
 					get(_target, key) {
-						const localisation = $json(key.toString(), options.locale);
+						const keyStr = key.toString();
+						const current = $json(keyStr, options.locale);
+						const currentMissing = current === keyStr || current == null;
+
+						let resolved: unknown;
+						if (options.locale === FALLBACK_LOCALE) {
+							if (currentMissing) return undefined;
+							resolved = current;
+						} else {
+							const fallback = $json(keyStr, FALLBACK_LOCALE);
+							const fallbackMissing =
+								fallback === keyStr || fallback == null;
 
-						if (localisation === key.toString()) return undefined;
+							if (currentMissing && fallbackMissing) return undefined;
+							if (currentMissing) resolved = fallback;
+							else if (fallbackMissing) resolved = current;
+							else resolved = deepMerge(fallback, current);
+						}
 
 						const replaceValues = (
 							localisation: InterpolationValues,
@@ -45,14 +76,14 @@ const createLocale = () => {
 
 							const updatedLocalisation: InterpolationValues = {};
 
-							for (const [key, value] of Object.entries(localisation)) {
+							for (const [k, value] of Object.entries(localisation)) {
 								if (typeof value === "string") {
-									updatedLocalisation[key] = value.replace(
+									updatedLocalisation[k] = value.replace(
 										/\{(\w+)\}/g,
 										(match, name) => (values ? values[name] : match) as string,
 									);
 								} else {
-									updatedLocalisation[key] = replaceValues(
+									updatedLocalisation[k] = replaceValues(
 										value as InterpolationValues,
 										values,
 									) as InterpolationValue;
@@ -64,11 +95,11 @@ const createLocale = () => {
 
 						if (options.values)
 							return replaceValues(
-								localisation as unknown as InterpolationValues,
+								resolved as unknown as InterpolationValues,
 								options.values,
 							);
 
-						return localisation;
+						return resolved;
 					},
 				},
 			);