diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index cfd3fcf8..e7abc8dc 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -169,4 +169,4 @@ jobs:
           while ! nc -z localhost 8080 </dev/null; do echo Waiting for PHP server to start...; sleep 1; done
         timeout-minutes: 1
       - name: Run behat tests
-        run: ./bin/behat -fprogress --no-interaction --profile=${{ matrix.profile }}
+        run: ./bin/behat -fprogress --no-interaction --profile=${{ matrix.profile }} --strict
diff --git a/src/Json/Json.php b/src/Json/Json.php
index f8694474..9f954253 100644
--- a/src/Json/Json.php
+++ b/src/Json/Json.php
@@ -4,7 +4,7 @@
 
 use Symfony\Component\PropertyAccess\PropertyAccessor;
 
-class Json
+class Json implements \Stringable
 {
     protected $content;
 
@@ -45,7 +45,7 @@ public function encode($pretty = true)
         return json_encode($this->content, $flags);
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->encode(false);
     }
diff --git a/src/Xml/Dom.php b/src/Xml/Dom.php
index 2b3488e7..f6de33d8 100644
--- a/src/Xml/Dom.php
+++ b/src/Xml/Dom.php
@@ -2,7 +2,7 @@
 
 namespace Behatch\Xml;
 
-class Dom
+class Dom implements \Stringable
 {
     private $dom;
 
@@ -16,7 +16,7 @@ public function __construct($content)
         $this->throwError();
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         $this->dom->formatOutput = true;
 
diff --git a/tests/features/browser.feature b/tests/features/en/browser.feature
similarity index 99%
rename from tests/features/browser.feature
rename to tests/features/en/browser.feature
index b95375d3..46b932f7 100644
--- a/tests/features/browser.feature
+++ b/tests/features/en/browser.feature
@@ -1,3 +1,4 @@
+# language: en
 Feature: Browser Feature
 
     # If this scenario fails
diff --git a/tests/features/debug.feature b/tests/features/en/debug.feature
similarity index 96%
rename from tests/features/debug.feature
rename to tests/features/en/debug.feature
index 81990716..ed75d43a 100644
--- a/tests/features/debug.feature
+++ b/tests/features/en/debug.feature
@@ -1,3 +1,4 @@
+# language: en
 Feature: Browser Feature
 
     @user
diff --git a/tests/features/json.feature b/tests/features/en/json.feature
similarity index 99%
rename from tests/features/json.feature
rename to tests/features/en/json.feature
index df6685c3..e2874b81 100644
--- a/tests/features/json.feature
+++ b/tests/features/en/json.feature
@@ -1,3 +1,4 @@
+# language: en
 @json
 Feature: Testing JSONContext
 
diff --git a/tests/features/rest.feature b/tests/features/en/rest.feature
similarity index 98%
rename from tests/features/rest.feature
rename to tests/features/en/rest.feature
index 0b0a93c2..a08054ae 100644
--- a/tests/features/rest.feature
+++ b/tests/features/en/rest.feature
@@ -1,3 +1,4 @@
+# language: en
 @rest
 Feature: Testing RESTContext
 
@@ -100,7 +101,7 @@ Feature: Testing RESTContext
 
     Scenario: Accept header should not be set by dfault
       When I send a GET request to "/rest/index.php"
-      Then I should not see "HTTP_ACCEPT"
+      Then I should not see "HTTP_ACCEPT :"
 
   @>php5.5
     Scenario: Set content headers in POST request
diff --git a/tests/features/system.feature b/tests/features/en/system.feature
similarity index 98%
rename from tests/features/system.feature
rename to tests/features/en/system.feature
index f516360a..88214d1d 100644
--- a/tests/features/system.feature
+++ b/tests/features/en/system.feature
@@ -1,3 +1,4 @@
+# language: en
 Feature: System feature
 
     Scenario: Testing execution
diff --git a/tests/features/table.feature b/tests/features/en/table.feature
similarity index 99%
rename from tests/features/table.feature
rename to tests/features/en/table.feature
index 6584624e..a0eaf4d0 100644
--- a/tests/features/table.feature
+++ b/tests/features/en/table.feature
@@ -1,3 +1,4 @@
+# language: en
 Feature: Browser Feature
 
     Scenario: Testing access to /table/index.html
diff --git a/tests/features/xml.feature b/tests/features/en/xml.feature
similarity index 99%
rename from tests/features/xml.feature
rename to tests/features/en/xml.feature
index 24645c2d..d8e02d2b 100644
--- a/tests/features/xml.feature
+++ b/tests/features/en/xml.feature
@@ -1,3 +1,4 @@
+# language: en
 @xml
 Feature: Testing XmlContext
 
diff --git a/tests/features/ja/browser.feature b/tests/features/ja/browser.feature
index 4dad9445..bedde7b1 100644
--- a/tests/features/ja/browser.feature
+++ b/tests/features/ja/browser.feature
@@ -9,7 +9,7 @@
     @javascript
     シナリオ: Testing simple web access
        前提 "/index.html" を表示している
-        ならば　画面に "Congratulations, you've correctly set up your apache environment." と表示されていること
+        ならば 画面に "Congratulations, you've correctly set up your apache environment." と表示されていること
 
     @statusCode
     シナリオ: Basic authentication
@@ -65,9 +65,9 @@
         ならば 私が"timeout"を見るまで3秒間待つ
         かつ 私が1秒間待つ
         かつ 私が"#iframe"要素を見るまで待つ
-        かつ　私が "#iframe" 要素を見るまで 5 秒間待つ
-        かつ　私が "#iframe" 要素を見るまで 5 秒待つ
-        かつ　"#iframe" 要素を見るまで 5 秒待つ
+        かつ 私が "#iframe" 要素を見るまで 5 秒間待つ
+        かつ 私が "#iframe" 要素を見るまで 5 秒待つ
+        かつ "#iframe" 要素を見るまで 5 秒待つ
 
     @javascript
     シナリオ: Check element visibility