diff --git a/deployment/docker/error_403.html b/deployment/docker/error_403.html
index 107e6b38198e67e5fc26db62cfdf985c06143925..1239488d45946910bdb921ff730f04644c0fe7bc 100644
--- a/deployment/docker/error_403.html
+++ b/deployment/docker/error_403.html
@@ -1,4 +1,4 @@
<!DOCTYPE html>
-<html>
+<html lang="en">
<body>You don't have access to this location.</body>
</html>
diff --git a/deployment/docker/error_404.html b/deployment/docker/error_404.html
index 82dcae12e2d74d13e67f5d1dcbe3b9e571e7b596..b9ef2ebcf96de09f9a054f2ff63e57301fc72554 100644
--- a/deployment/docker/error_404.html
+++ b/deployment/docker/error_404.html
@@ -1,4 +1,4 @@
<!DOCTYPE html>
-<html>
+<html lang="en">
<body>This is not the page you're looking for.</body>
</html>
diff --git a/deployment/docker/error_50x.html b/deployment/docker/error_50x.html
index 5cab55bde5f4caa17b1dcbcbdbca9f3d0a8b8cab..e96d64eda8d63e5b6a42b8422415d7079829cbad 100644
--- a/deployment/docker/error_50x.html
+++ b/deployment/docker/error_50x.html
@@ -1,4 +1,4 @@
<!DOCTYPE html>
-<html>
+<html lang="en">
<body>Please stand by, application will be available again shortly.</body>
</html>
diff --git a/deployment/docker/index.html b/deployment/docker/index.html
index a1ed4aa57bdc22572e7a4ab93cf1312cd06d915b..c1afa7cd1f46793fd68785e0bf73f8c8cb5a24e2 100644
--- a/deployment/docker/index.html
+++ b/deployment/docker/index.html
@@ -1,4 +1,4 @@
<!DOCTYPE html>
-<html>
+<html lang="en">
<body>There is nothing to be seen here.</body>
</html>
diff --git a/src/backoffice/templates/backoffice/base.html b/src/backoffice/templates/backoffice/base.html
index 6f0571e4a156312c29ea037c18959ca339708c22..afda8cb2fe5a564c5726e9ddfcd0129051727ec3 100644
--- a/src/backoffice/templates/backoffice/base.html
+++ b/src/backoffice/templates/backoffice/base.html
@@ -2,7 +2,7 @@
{% load static %}
{% trans "nav_activetab_srmarker" as activetab_srmarker %}
<!DOCTYPE html>
-<html>
+<html lang="{{ request.LANGUAGE_CODE | default:"en" }}">
<head>
<title>
{% block title %}
diff --git a/src/core/templates/core/assembly_notification.html b/src/core/templates/core/assembly_notification.html
index bfd1bd3eed6b19508f4bf5450413ac19dadc3e3c..75bbedc3632e8776ac22a78cfccf2b60a0249744 100644
--- a/src/core/templates/core/assembly_notification.html
+++ b/src/core/templates/core/assembly_notification.html
@@ -1,5 +1,5 @@
<!DOCTYPE html>
-<html>
+<html lang="de">
<head>
<title>{{ subject }}</title>
</head>
diff --git a/src/core/templates/core/directmessage.html b/src/core/templates/core/directmessage.html
index fff718c1e3000807049067d90182c9e85a30f59b..1f0ef673e7f0ed685228345b9385c5b280971055 100644
--- a/src/core/templates/core/directmessage.html
+++ b/src/core/templates/core/directmessage.html
@@ -1,5 +1,5 @@
<!DOCTYPE html5>
-<html>
+<html lang="de">
<body>
{% if conference != None %}
<h1>
diff --git a/src/core/templates/oauth2_provider/authorize.html b/src/core/templates/oauth2_provider/authorize.html
index bc572959503aeaeb3b0bc7fd78decae1cee11954..af49e6903199d6de0151a7720b4b474809493ce5 100644
--- a/src/core/templates/oauth2_provider/authorize.html
+++ b/src/core/templates/oauth2_provider/authorize.html
@@ -1,5 +1,5 @@
<!DOCTYPE html>
-<html>
+<html lang="en">
<head>
<title>Hub SSO</title>
<style>
diff --git a/src/core/templates/oauth2_provider/out-of-band-display-token.html b/src/core/templates/oauth2_provider/out-of-band-display-token.html
index fe86066a829fdec2f66f9b738b47cca03a86faac..5e381b614a91a6b5c10006021a6864266d1eb489 100644
--- a/src/core/templates/oauth2_provider/out-of-band-display-token.html
+++ b/src/core/templates/oauth2_provider/out-of-band-display-token.html
@@ -1,5 +1,5 @@
<!DOCTYPE html>
-<html>
+<html lang="end">
<head>
<title>Hub SSO</title>
<style>
diff --git a/src/plainui/jinja2/plainui/metanav.html.j2 b/src/plainui/jinja2/plainui/metanav.html.j2
index a530c226dcaa61984ccd632bfd2daa939d99cb86..9e434152de161f1a748002e8d1823d78a98f2c8a 100644
--- a/src/plainui/jinja2/plainui/metanav.html.j2
+++ b/src/plainui/jinja2/plainui/metanav.html.j2
@@ -1,6 +1,6 @@
{% if include_header %}
<!DOCTYPE html>
- <html>
+ <html lang="en">
<head>
<title>{{ conf.slug }} Meta Navigation</title>
</head>
diff --git a/src/plainui/static/error_pages/403.html b/src/plainui/static/error_pages/403.html
index 5084ab47439f26796b9af999a33619fd6465d969..3dc29f434253ee93493fc30f5d20b68878e2258a 100644
--- a/src/plainui/static/error_pages/403.html
+++ b/src/plainui/static/error_pages/403.html
@@ -1,5 +1,5 @@
<!DOCTYPE html>
-<html>
+<html lang="en">
<head>
<title>403 - Forbidden</title>
<style type="text/css">
diff --git a/src/plainui/static/error_pages/404.html b/src/plainui/static/error_pages/404.html
index 869911e13a26c912f06917eba64bfd9ed4d44a97..3eabccd38345abd82060ace907b1dc9d0f02277a 100644
--- a/src/plainui/static/error_pages/404.html
+++ b/src/plainui/static/error_pages/404.html
@@ -1,5 +1,5 @@
<!DOCTYPE html>
-<html>
+<html lang="en">
<head>
<title>404 - OOOOOPS (not found)</title>
<style type="text/css">
diff --git a/src/plainui/static/error_pages/500.html b/src/plainui/static/error_pages/500.html
index 313f0bb3533d754ba4bf09d4a8624c3fd8fcc48b..125def779ec16b9d64bb8d7ae2f754f815b85a4e 100644
--- a/src/plainui/static/error_pages/500.html
+++ b/src/plainui/static/error_pages/500.html
@@ -1,5 +1,5 @@
<!DOCTYPE html>
-<html>
+<html lang="en">
<head>
<title>500 - Internal Server Error</title>
<style type="text/css">
diff --git a/src/plainui/templates/403.html b/src/plainui/templates/403.html
index 7e2594a1fafb88a369b0fbc2206c9eb301fa1797..23f9e86820e93fc4dbe5d528db88e3c211d37acd 100644
--- a/src/plainui/templates/403.html
+++ b/src/plainui/templates/403.html
@@ -1,6 +1,6 @@
{% load static %}
<!DOCTYPE html>
-<html>
+<html lang="en">
<head>
<title>403 - Forbidden</title>
<style type="text/css">
diff --git a/src/plainui/templates/404.html b/src/plainui/templates/404.html
index c2064bfa9dfcea5bca5f1a6fd5180f4d3344e084..7ab1dc90cfcedf326b55e3b0baa4f622842d5df2 100644
--- a/src/plainui/templates/404.html
+++ b/src/plainui/templates/404.html
@@ -1,6 +1,6 @@
{% load static %}
<!DOCTYPE html>
-<html>
+<html lang="en">
<head>
<title>404 - OOOOOPS (not found)</title>
<style type="text/css">
diff --git a/src/plainui/templates/500.html b/src/plainui/templates/500.html
index b54aff74800e650b9a4ef73065048ada11520b5f..4f13dae2b1642f47a58a3c4cace2658325c8600c 100644
--- a/src/plainui/templates/500.html
+++ b/src/plainui/templates/500.html
@@ -1,6 +1,6 @@
{% load static %}
<!DOCTYPE html>
-<html>
+<html lang="en">
<head>
<title>500 - Internal Server Error</title>
<style type="text/css">