default language check

src/documents/__init__.py

@@ -1 +1,2 @@
-from .checks import changed_password_check
+# this is here so that django finds the checks.
+from .checks import *

src/paperless_tesseract/__init__.py

@@ -0,0 +1,2 @@
+# this is here so that django finds the checks.
+from .checks import *

src/paperless_tesseract/checks.py

@@ -0,0 +1,24 @@
+import subprocess
+
+from django.conf import settings
+from django.core.checks import Error, register
+
+
+def get_tesseract_langs():
+    with subprocess.Popen(['tesseract', '--list-langs'], stdout=subprocess.PIPE) as p:
+        stdout, stderr = p.communicate()
+
+    return stdout.decode().strip().split("\n")[1:]
+
+
+@register()
+def check_default_language_available(app_configs, **kwargs):
+    langs = get_tesseract_langs()
+
+    if not settings.OCR_LANGUAGE in langs:
+        return [Error(
+            f"The default ocr language {settings.OCR_LANGUAGE} is "
+            f"not installed. Paperless cannot OCR your documents "
+            f"without it. Please fix PAPERLESS_OCR_LANGUAGE.")]
+    else:
+        return []