diff --git a/src/shared/components/comment/comment-form.tsx b/src/shared/components/comment/comment-form.tsx
index 0226e23e..5cce2b53 100644
--- a/src/shared/components/comment/comment-form.tsx
+++ b/src/shared/components/comment/comment-form.tsx
@@ -41,10 +41,12 @@ export class CommentForm extends Component<CommentFormProps, any> {
           : undefined
         : undefined;
 
-    const userInterfaceLangId = getUserInterfaceLangId({
-      myUserInfo: UserService.Instance.myUserInfo,
-      allLanguages: this.props.allLanguages,
-    });
+    const userInterfaceLangId = !UserService.Instance.myUserInfo
+      ? 0
+      : getUserInterfaceLangId({
+          myUserInfo: UserService.Instance.myUserInfo,
+          allLanguages: this.props.allLanguages,
+        });
 
     return (
       <div
diff --git a/src/shared/components/common/markdown-textarea.tsx b/src/shared/components/common/markdown-textarea.tsx
index 97d1d142..119913d3 100644
--- a/src/shared/components/common/markdown-textarea.tsx
+++ b/src/shared/components/common/markdown-textarea.tsx
@@ -260,8 +260,12 @@ export class MarkdownTextArea extends Component<
               <LanguageSelect
                 iconVersion
                 allLanguages={this.props.allLanguages}
+                // Only set the selected language ID if it exists as an option
+                // in the dropdown; otherwise, set it to 0 (Undetermined)
                 selectedLanguageIds={
-                  languageId ? Array.of(languageId) : undefined
+                  languageId && this.props.siteLanguages.includes(languageId)
+                    ? Array.of(languageId)
+                    : [0]
                 }
                 siteLanguages={this.props.siteLanguages}
                 onChange={this.handleLanguageChange}
diff --git a/src/shared/components/post/post-form.tsx b/src/shared/components/post/post-form.tsx
index f14eac73..780ea797 100644
--- a/src/shared/components/post/post-form.tsx
+++ b/src/shared/components/post/post-form.tsx
@@ -326,10 +326,12 @@ export class PostForm extends Component<PostFormProps, PostFormState> {
   render() {
     const url = this.state.form.url;
 
-    const userInterfaceLangId = getUserInterfaceLangId({
-      myUserInfo: UserService.Instance.myUserInfo,
-      allLanguages: this.props.allLanguages,
-    });
+    const userInterfaceLangId = !UserService.Instance.myUserInfo
+      ? 0
+      : getUserInterfaceLangId({
+          myUserInfo: UserService.Instance.myUserInfo,
+          allLanguages: this.props.allLanguages,
+        });
 
     return (
       <form className="post-form" onSubmit={linkEvent(this, handlePostSubmit)}>
diff --git a/src/shared/utils/app/user-interface-language.ts b/src/shared/utils/app/user-interface-language.ts
index 5ccd61c3..707526d7 100644
--- a/src/shared/utils/app/user-interface-language.ts
+++ b/src/shared/utils/app/user-interface-language.ts
@@ -17,6 +17,6 @@ export default function getUserInterfaceLanguageId({
     return i18nLang.indexOf(lang.code) === 0;
   });
 
-  // Return the ID of that language object
+  // Return the ID of that language object, or "0" for Undetermined
   return userLang.id || 0;
 }