diff --git a/CHANGES.rst b/CHANGES.rst
index 2d367074..1099ccc2 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -18,6 +18,13 @@ Breaking Changes
 
 - TBD
 
+Bugfixes & Improvements
+^^^^^^^^^^^^^^^^^^^^^^^
+
+- Make language code handling more robust (`#983`_, ix5)
+
+.. _#983: https://github.com/posativ/isso/pull/983
+
 New Features
 ^^^^^^^^^^^^
 
diff --git a/isso/js/app/config.js b/isso/js/app/config.js
index fef089c0..71d5f569 100644
--- a/isso/js/app/config.js
+++ b/isso/js/app/config.js
@@ -17,9 +17,21 @@ for (var i = 0; i < js.length; i++) {
         var attr = js[i].attributes[j];
         if (/^data-isso-/.test(attr.name)) {
             try {
-                config[attr.name.substring(10)] = JSON.parse(attr.value);
+                // Normalize underscores to dashes so that language-specific
+                // strings can be caught better later on,
+                // e.g. data-isso-postbox-text-text-PT_BR becomes
+                // postbox-text-text-pt-br.
+                // Also note that attr.name only gives lowercase strings as per
+                // HTML spec, e.g. data-isso-FOO-Bar becomes foo-bar, but since
+                // the test environment's jest-environment-jsdom seemingly does
+                // not follow that convention, convert to lowercase here anyway.
+                config[attr.name.substring(10)
+                       .replace(/_/g, '-')
+                       .toLowerCase()] = JSON.parse(attr.value);
             } catch (ex) {
-                config[attr.name.substring(10)] = attr.value;
+                config[attr.name.substring(10)
+                       .replace(/_/g, '-')
+                       .toLowerCase()] = attr.value;
             }
         }
     }
diff --git a/isso/js/app/i18n.js b/isso/js/app/i18n.js
index 304b9eca..25794080 100644
--- a/isso/js/app/i18n.js
+++ b/isso/js/app/i18n.js
@@ -133,7 +133,10 @@ if (!plural || !translations) {
 }
 
 var translate = function(msgid) {
-    return config[msgid + '-text-' + lang] ||
+    // Need to convert the language string to lowercase because data-isso-*
+    // attributes are automatically cast to lowercase as per HTML spec
+    // https://stackoverflow.com/questions/36176474/camel-case-in-html-tag-attributes-and-jquery-doesnt-work-why
+    return config[msgid + '-text-' + lang.toLowerCase()] ||
       translations[msgid] ||
       catalogue.en[msgid] ||
       "[?" + msgid + "]";
diff --git a/isso/js/tests/unit/i18n-overrides.test.js b/isso/js/tests/unit/i18n-overrides.test.js
new file mode 100644
index 00000000..1df1e6fd
--- /dev/null
+++ b/isso/js/tests/unit/i18n-overrides.test.js
@@ -0,0 +1,113 @@
+/**
+ * @jest-environment jsdom
+ */
+
+/* Keep the above exactly as-is!
+ * https://jestjs.io/docs/configuration#testenvironment-string
+ * https://jestjs.io/docs/configuration#testenvironmentoptions-object
+ */
+
+"use strict";
+
+beforeEach(() => {
+  jest.resetModules();
+  document.body.innerHTML = '';
+});
+
+test('data-isso-* attributes should override i18n strings', () => {
+  // Set up our document body
+  document.body.innerHTML =
+    '<div id=isso-thread></div>' +
+    // Note: `src` and `data-isso` need to be set,
+    // else `api` fails to initialize!
+    '<script src="http://isso.api/js/embed.min.js"'
+          + 'data-isso="/"'
+          + 'data-isso-lang="de"'
+          + 'data-isso-postbox-text-text-de="Kommentiere hier, Alter!"'
+          + '</script>';
+
+  const isso = require("app/isso");
+  const $ = require("app/dom");
+  const config = require("app/config");
+  const template = require("app/template");
+  const i18n = require("app/i18n");
+  const svg = require("app/svg");
+
+  template.set("conf", config);
+  template.set("i18n", i18n.translate);
+  template.set("pluralize", i18n.pluralize);
+  template.set("svg", svg);
+
+  var isso_thread = $('#isso-thread');
+  isso_thread.append('<div id="isso-root"></div>');
+  isso_thread.append(new isso.Postbox(null));
+
+  expect($('.isso-textarea').placeholder).toMatch('Kommentiere hier, Alter!');
+});
+
+test('data-isso-* i18n strings should be accepted with dashes', () => {
+  document.body.innerHTML =
+    '<div id=isso-thread></div>' +
+    // Note: `src` and `data-isso` need to be set,
+    // else `api` fails to initialize!
+    '<script src="http://isso.api/js/embed.min.js"'
+          + 'data-isso="/"></script>';
+
+  var script_tag = document.getElementsByTagName('script')[0];
+  script_tag.setAttributeNS(null, 'data-isso-lang', 'pt_BR');
+  script_tag.setAttributeNS(null, 'data-isso-postbox-text-text-pt-br', 'Digite seu comentário.');
+
+  expect(script_tag.getAttribute('data-isso-lang')).toBe('pt_BR');
+  // expect(script_tag).toBe('')
+
+  const isso = require("app/isso");
+  const $ = require("app/dom");
+  const config = require("app/config");
+  const template = require("app/template");
+  const i18n = require("app/i18n");
+  const svg = require("app/svg");
+
+  template.set("conf", config);
+  template.set("i18n", i18n.translate);
+  template.set("pluralize", i18n.pluralize);
+  template.set("svg", svg);
+
+  var isso_thread = $('#isso-thread');
+  isso_thread.append('<div id="isso-root"></div>');
+  isso_thread.append(new isso.Postbox(null));
+
+  expect($('.isso-textarea').placeholder).toMatch('Digite seu comentário.');
+});
+
+test('data-isso-* i18n strings should be accepted with underscores', () => {
+  document.body.innerHTML =
+    '<div id=isso-thread></div>' +
+    // Note: `src` and `data-isso` need to be set,
+    // else `api` fails to initialize!
+    '<script src="http://isso.api/js/embed.min.js"'
+          + 'data-isso="/"></script>';
+
+  var script_tag = document.getElementsByTagName('script')[0];
+  script_tag.setAttributeNS(null, 'data-isso-lang', 'pt_br');
+  script_tag.setAttributeNS(null, 'data-isso-postbox-text-text-pt_BR', 'Digite seu comentário.');
+
+  expect(script_tag.getAttribute('data-isso-lang')).toBe('pt_br');
+
+  const isso = require("app/isso");
+  const $ = require("app/dom");
+  const config = require("app/config");
+  const template = require("app/template");
+  const i18n = require("app/i18n");
+  const svg = require("app/svg");
+
+  template.set("conf", config);
+  template.set("i18n", i18n.translate);
+  template.set("pluralize", i18n.pluralize);
+  template.set("svg", svg);
+
+  var isso_thread = $('#isso-thread');
+  isso_thread.append('<div id="isso-root"></div>');
+  isso_thread.append(new isso.Postbox(null));
+
+  expect($('.isso-textarea').placeholder).toMatch('Digite seu comentário.');
+});