diff --git a/eslint/locale.js b/eslint/locale.js
new file mode 100644
index 0000000000..b6f6f762be
--- /dev/null
+++ b/eslint/locale.js
@@ -0,0 +1,145 @@
+/* This is a ESLint rule to report use of the `i18n.ts` and `i18n.tsx`
+ * objects that reference translation items that don't actually exist
+ * in the lexicon (the `locale/` files)
+ */
+
+/* given a MemberExpression node, collects all the member names
+ *
+ * e.g. for a bit of code like `foo=one.two.three`, `collectMembers`
+ * called on the node for `three` would return `['one', 'two',
+ * 'three']`
+ */
+function collectMembers(node) {
+	if (!node) return [];
+	if (node.type !== 'MemberExpression') return [];
+	return [ node.property.name, ...collectMembers(node.parent) ];
+}
+
+/* given an object and an array of names, recursively descends the
+ * object via those names
+ *
+ * e.g. `walkDown({one:{two:{three:15}}},['one','two','three'])` would
+ * return 15
+ */
+function walkDown(locale, path) {
+	if (!locale) return null;
+	if (!path || path.length === 0) return locale;
+	return walkDown(locale[path[0]], path.slice(1));
+}
+
+/* given a MemberExpression node, returns its attached CallExpression
+ * node if present
+ *
+ * e.g. for a bit of code like `foo=one.two.three()`,
+ * `findCallExpression` called on the node for `three` would return
+ * the node for function call (which is the parent of the `one` and
+ * `two` nodes, and holds the nodes for the argument list)
+ *
+ * if the code had been `foo=one.two.three`, `findCallExpression`
+ * would have returned null, because there's no function call attached
+ * to the MemberExpressions
+ */
+function findCallExpression(node) {
+	if (node.type === 'CallExpression') return node
+	if (node.parent?.type === 'CallExpression') return node.parent;
+	if (node.parent?.type === 'MemberExpression') return findCallExpression(node.parent);
+	return null;
+}
+
+/* the actual rule body
+ */
+function theRule(context) {
+	// we get the locale/translations via the options; it's the data
+	// that goes into a specific language's JSON file, see
+	// `scripts/build-assets.mjs`
+	const locale = context.options[0];
+	return {
+		// for all object member access that have an identifier 'i18n'...
+		'MemberExpression:has(> Identifier[name=i18n])': (node) => {
+			// sometimes we get MemberExpression nodes that have a
+			// *descendent* with the right identifier: skip them, we'll get
+			// the right ones as well
+			if (node.object?.name != 'i18n') {
+				return;
+			}
+
+			// `method` is going to be `'ts'` or `'tsx'`, `path` is going to
+			// be the various translation steps/names
+			const [ method, ...path ] = collectMembers(node);
+			const pathStr = `i18n.${method}.${path.join('.')}`;
+
+			// does that path point to a real translation?
+			const matchingNode = walkDown(locale, path);
+			if (!matchingNode) {
+				context.report({
+					node,
+					message: `translation missing for ${pathStr}`,
+				});
+				return;
+			}
+
+			// some more checks on how the translation is called
+			if (method == 'ts') {
+				if (matchingNode.match(/\{/)) {
+					context.report({
+						node,
+						message: `translation for ${pathStr} is parametric, but called via 'ts'`,
+					});
+					return;
+				}
+
+				if (findCallExpression(node)) {
+					context.report({
+						node,
+						message: `translation for ${pathStr} is not parametric, but is called as a function`,
+					});
+				}
+			}
+
+			if (method == 'tsx') {
+				if (!matchingNode.match(/\{/)) {
+					context.report({
+						node,
+						message: `translation for ${pathStr} is not parametric, but called via 'tsx'`,
+					});
+					return;
+				}
+
+				const callExpression = findCallExpression(node);
+
+				if (!callExpression) {
+					context.report({
+						node,
+						message: `translation for ${pathStr} is parametric, but not called as a function`,
+					});
+					return;
+				}
+
+				const parameterCount = [...matchingNode.matchAll(/\{/g)].length ?? 0;
+				const argumentCount = callExpression.arguments.length;
+				if (parameterCount !== argumentCount) {
+					context.report({
+						node,
+						message: `translation for ${pathStr} has ${parameterCount} parameters, but is called with ${argumentCount} arguments`,
+					});
+					return;
+				}
+			}
+		},
+	};
+}
+
+module.exports = {
+	meta: {
+		type: 'problem',
+		docs: {
+			description: 'assert that all translations used are present in the locale files',
+		},
+		schema: [
+			// here we declare that we need the locale/translation as a
+			// generic object
+			{ type: 'object', additionalProperties: true },
+		],
+	},
+	create: theRule,
+};
diff --git a/eslint/locale.test.js b/eslint/locale.test.js
new file mode 100644
index 0000000000..cf64961054
--- /dev/null
+++ b/eslint/locale.test.js
@@ -0,0 +1,29 @@
+const {RuleTester} = require("eslint");
+const localeRule = require("./locale");
+
+const locale = { foo: { bar: 'ok', baz: 'good {x}' }, top: '123' };
+
+const ruleTester = new RuleTester();
+
+ruleTester.run(
+  'sharkey-locale',
+  localeRule,
+  {
+    valid: [
+      {code: 'i18n.ts.foo.bar', options: [locale] },
+      {code: 'i18n.ts.top', options: [locale] },
+      {code: 'i18n.tsx.foo.baz(1)', options: [locale] },
+      {code: 'whatever.i18n.ts.blah.blah', options: [locale] },
+      {code: 'whatever.i18n.tsx.does.not.matter', options: [locale] },
+    ],
+    invalid: [
+      {code: 'i18n.ts.not', options: [locale], errors: 1 },
+      {code: 'i18n.tsx.deep.not', options: [locale], errors: 1 },
+      {code: 'i18n.tsx.deep.not(12)', options: [locale], errors: 1 },
+      {code: 'i18n.tsx.top(1)', options: [locale], errors: 1 },
+      {code: 'i18n.ts.foo.baz', options: [locale], errors: 1 },
+      {code: 'i18n.tsx.foo.baz', options: [locale], errors: 1 },
+    ],
+  },
+);
+
diff --git a/packages/frontend/eslint.config.js b/packages/frontend/eslint.config.js
index 28796e8d6b..2841a5592a 100644
--- a/packages/frontend/eslint.config.js
+++ b/packages/frontend/eslint.config.js
@@ -4,6 +4,8 @@ import parser from 'vue-eslint-parser';
 import pluginVue from 'eslint-plugin-vue';
 import pluginMisskey from '@misskey-dev/eslint-plugin';
 import sharedConfig from '../shared/eslint.config.js';
+import localeRule from '../../eslint/locale.js';
+import { build as buildLocales } from '../../locales/index.js';
 
 export default [
 	...sharedConfig,
@@ -14,6 +16,7 @@ export default [
 	...pluginVue.configs['flat/recommended'],
 	{
 		files: ['{src,test,js,@types}/**/*.{ts,vue}'],
+		plugins: { sharkey: { rules: { locale: localeRule } } },
 		languageOptions: {
 			globals: {
 				...Object.fromEntries(Object.entries(globals.node).map(([key]) => [key, 'off'])),
@@ -44,6 +47,8 @@ export default [
 			},
 		},
 		rules: {
+			'sharkey/locale': ['error', buildLocales()['ja-JP']],
+
 			'@typescript-eslint/no-empty-interface': ['error', {
 				allowSingleExtends: true,
 			}],