diff --git a/frontend/src/Settings/Profiles/Metadata/EditMetadataProfileModalContent.js b/frontend/src/Settings/Profiles/Metadata/EditMetadataProfileModalContent.js
index 71fc7a7a0..1099e9949 100644
--- a/frontend/src/Settings/Profiles/Metadata/EditMetadataProfileModalContent.js
+++ b/frontend/src/Settings/Profiles/Metadata/EditMetadataProfileModalContent.js
@@ -173,6 +173,7 @@ function EditMetadataProfileModalContent(props) {
                   type={inputTypes.TEXT}
                   name="allowedLanguages"
                   {...allowedLanguages}
+                  helpText={translate('Iso639-3')}
                   onChange={onInputChange}
                 />
               </FormGroup>
diff --git a/src/NzbDrone.Core/Books/Calibre/Extensions.cs b/src/NzbDrone.Core/Books/Calibre/Extensions.cs
index 8b6c0048b..9b3aa10a9 100644
--- a/src/NzbDrone.Core/Books/Calibre/Extensions.cs
+++ b/src/NzbDrone.Core/Books/Calibre/Extensions.cs
@@ -13,12 +13,16 @@ namespace NzbDrone.Core.Books.Calibre
         private static readonly Dictionary<string, string> ByThree;
         private static readonly Dictionary<string, string> NameMap;
 
+        public static HashSet<string> KnownLanguages { get; }
+
         static Extensions()
         {
             var assembly = Assembly.GetExecutingAssembly();
             TwoToThree = InitializeDictionary(assembly, "2to3.json");
             ByThree = InitializeDictionary(assembly, "by3.json");
             NameMap = InitializeDictionary(assembly, "name_map.json");
+
+            KnownLanguages = ByThree.Keys.ToHashSet();
         }
 
         private static Dictionary<string, string> InitializeDictionary(Assembly assembly, string resource)
diff --git a/src/NzbDrone.Core/Localization/Core/en.json b/src/NzbDrone.Core/Localization/Core/en.json
index 0d3ebd1b3..e3e025fb1 100644
--- a/src/NzbDrone.Core/Localization/Core/en.json
+++ b/src/NzbDrone.Core/Localization/Core/en.json
@@ -305,6 +305,7 @@
   "IsExpandedShowFileInfo": "Show file info",
   "IsInUseCantDeleteAMetadataProfileThatIsAttachedToAnAuthorOrImportList": "Can't delete a metadata profile that is attached to an author or import list",
   "IsInUseCantDeleteAQualityProfileThatIsAttachedToAnAuthorOrImportList": "Can't delete a quality profile that is attached to an author or import list",
+  "Iso639-3": "ISO 639-3 language codes, or 'null', comma separated",
   "IsShowingMonitoredMonitorSelected": "Monitor Selected",
   "IsShowingMonitoredUnmonitorSelected": "Unmonitor Selected",
   "IsTagUsedCannotBeDeletedWhileInUse": "Cannot be deleted while in use",
diff --git a/src/NzbDrone.Core/Profiles/Metadata/MetadataProfileService.cs b/src/NzbDrone.Core/Profiles/Metadata/MetadataProfileService.cs
index 53afee3d1..c494f9244 100644
--- a/src/NzbDrone.Core/Profiles/Metadata/MetadataProfileService.cs
+++ b/src/NzbDrone.Core/Profiles/Metadata/MetadataProfileService.cs
@@ -5,6 +5,7 @@ using System.Text.RegularExpressions;
 using NLog;
 using NzbDrone.Common.Extensions;
 using NzbDrone.Core.Books;
+using NzbDrone.Core.Books.Calibre;
 using NzbDrone.Core.ImportLists;
 using NzbDrone.Core.Lifecycle;
 using NzbDrone.Core.MediaFiles;
@@ -173,14 +174,14 @@ namespace NzbDrone.Core.Profiles.Metadata
 
         private List<Edition> FilterEditions(IEnumerable<Edition> editions, List<Edition> localEditions, List<BookFile> localFiles, MetadataProfile profile)
         {
-            var allowedLanguages = profile.AllowedLanguages.IsNotNullOrWhiteSpace() ? new HashSet<string>(profile.AllowedLanguages.Split(',').Select(x => x.Trim().ToLower())) : new HashSet<string>();
+            var allowedLanguages = profile.AllowedLanguages.IsNotNullOrWhiteSpace() ? new HashSet<string>(profile.AllowedLanguages.Trim(',').Split(',').Select(x => x.CanonicalizeLanguage())) : new HashSet<string>();
 
             var hash = new HashSet<Edition>(editions);
 
             var localHash = new HashSet<string>(localEditions.Where(x => x.ManualAdd).Select(x => x.ForeignEditionId));
             localHash.UnionWith(localFiles.Select(x => x.Edition.Value.ForeignEditionId));
 
-            FilterByPredicate(hash, x => x.ForeignEditionId, localHash, profile, (x, p) => !allowedLanguages.Any() || allowedLanguages.Contains(x.Language?.ToLower() ?? "null"), "edition language not allowed");
+            FilterByPredicate(hash, x => x.ForeignEditionId, localHash, profile, (x, p) => !allowedLanguages.Any() || allowedLanguages.Contains(x.Language?.CanonicalizeLanguage() ?? "null"), "edition language not allowed");
             FilterByPredicate(hash, x => x.ForeignEditionId, localHash, profile, (x, p) => !p.SkipMissingIsbn || x.Isbn13.IsNotNullOrWhiteSpace() || x.Asin.IsNotNullOrWhiteSpace(), "isbn and asin is missing");
             FilterByPredicate(hash, x => x.ForeignEditionId, localHash, profile, (x, p) => !MatchesTerms(x.Title, p.Ignored), "contains ignored terms");
 
@@ -272,7 +273,7 @@ namespace NzbDrone.Core.Profiles.Metadata
                     MinPopularity = 350,
                     SkipMissingDate = true,
                     SkipPartsAndSets = true,
-                    AllowedLanguages = "eng, en-US, en-GB, null"
+                    AllowedLanguages = "eng, null"
                 });
             }
 
diff --git a/src/Readarr.Api.V1/Profiles/Metadata/MetadataProfileController.cs b/src/Readarr.Api.V1/Profiles/Metadata/MetadataProfileController.cs
index 785d7a202..aad55ecad 100644
--- a/src/Readarr.Api.V1/Profiles/Metadata/MetadataProfileController.cs
+++ b/src/Readarr.Api.V1/Profiles/Metadata/MetadataProfileController.cs
@@ -1,6 +1,8 @@
 using System.Collections.Generic;
+using System.Linq;
 using FluentValidation;
 using Microsoft.AspNetCore.Mvc;
+using NzbDrone.Common.Extensions;
 using NzbDrone.Core.Profiles.Metadata;
 using NzbDrone.Http.REST.Attributes;
 using Readarr.Http;
@@ -17,6 +19,14 @@ namespace Readarr.Api.V1.Profiles.Metadata
         {
             _profileService = profileService;
             SharedValidator.RuleFor(c => c.Name).NotEqual("None").WithMessage("'None' is a reserved profile name").NotEmpty();
+            SharedValidator.RuleFor(c => c.AllowedLanguages)
+                .Must(x => x
+                    .Trim(',')
+                    .Split(',')
+                    .Select(y => y.Trim())
+                    .All(y => y == "null" || NzbDrone.Core.Books.Calibre.Extensions.KnownLanguages.Contains(y)))
+                .When(x => x.AllowedLanguages.IsNotNullOrWhiteSpace())
+                .WithMessage("Unknown languages");
         }
 
         [RestPostById]