diff --git a/package.json b/package.json
index 0ee4c3e3316..fdfb43b0d89 100644
--- a/package.json
+++ b/package.json
@@ -29,12 +29,14 @@
   ],
   "bin": {
     "reskindex": "scripts/reskindex.js",
-    "matrix-gen-i18n": "scripts/gen-i18n.js"
+    "matrix-gen-i18n": "scripts/gen-i18n.js",
+    "matrix-prune-i18n": "scripts/prune-i18n.js"
   },
   "scripts": {
     "reskindex": "node scripts/reskindex.js -h header",
     "reskindex:watch": "node scripts/reskindex.js -h header -w",
     "i18n": "matrix-gen-i18n",
+    "prunei18n": "matrix-prune-i18n",
     "build": "npm run reskindex && babel src -d lib --source-maps --copy-files",
     "build:watch": "babel src -w -d lib --source-maps --copy-files",
     "emoji-data-strip": "node scripts/emoji-data-strip.js",
diff --git a/scripts/prune-i18n.js b/scripts/prune-i18n.js
new file mode 100755
index 00000000000..b4fe8d69f58
--- /dev/null
+++ b/scripts/prune-i18n.js
@@ -0,0 +1,68 @@
+#!/usr/bin/env node
+
+/*
+Copyright 2017 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/*
+ * Looks through all the translation files and removes any strings
+ * which don't appear in en_EN.json.
+ * Use this if you remove a translation, but merge any outstanding changes
+ * from weblate first or you'll need to resolve the conflict in weblate.
+ */
+
+const fs = require('fs');
+const path = require('path');
+
+const I18NDIR = 'src/i18n/strings';
+
+const enStringsRaw = JSON.parse(fs.readFileSync(path.join(I18NDIR, 'en_EN.json')));
+
+const enStrings = new Set();
+for (const str of Object.keys(enStringsRaw)) {
+    const parts = str.split('|');
+    if (parts.length > 1) {
+        enStrings.add(parts[0]);
+    } else {
+        enStrings.add(str);
+    }
+}
+
+for (const filename of fs.readdirSync(I18NDIR)) {
+    if (filename === 'en_EN.json') continue;
+    if (filename === 'basefile.json') continue;
+    if (!filename.endsWith('.json')) continue;
+
+    const trs = JSON.parse(fs.readFileSync(path.join(I18NDIR, filename)));
+    const oldLen = Object.keys(trs).length;
+    for (const tr of Object.keys(trs)) {
+        const parts = tr.split('|');
+        const trKey = parts.length > 1 ? parts[0] : tr;
+        if (!enStrings.has(trKey)) {
+            delete trs[tr];
+        }
+    }
+
+    const removed = oldLen - Object.keys(trs).length;
+    if (removed > 0) {
+        console.log(`${filename}: removed ${removed} translations`);
+        // XXX: This is totally relying on the impl serialising the JSON object in the
+        // same order as they were parsed from the file. JSON.stringify() has a specific argument
+        // that can be used to control the order, but JSON.parse() lacks any kind of equivalent.
+        // Empirically this does maintain the order on my system, so I'm going to leave it like
+        // this for now.
+        fs.writeFileSync(path.join(I18NDIR, filename), JSON.stringify(trs, undefined, 4) + "\n");
+    }
+}