diff --git a/crates/codebook-config/src/settings.rs b/crates/codebook-config/src/settings.rs
index 1a3e89e..fea468d 100644
--- a/crates/codebook-config/src/settings.rs
+++ b/crates/codebook-config/src/settings.rs
@@ -89,7 +89,7 @@ impl<'de> Deserialize<'de> for ConfigSettings {
         D: serde::Deserializer<'de>,
     {
         fn to_lowercase_vec(v: Vec<String>) -> Vec<String> {
-            v.into_iter().map(|s| s.to_ascii_lowercase()).collect()
+            v.into_iter().map(|s| s.to_lowercase()).collect()
         }
         #[derive(Deserialize)]
         struct Helper {
@@ -200,7 +200,7 @@ impl ConfigSettings {
 
     /// Insert a word into the allowlist, returning true when it was newly added.
     pub fn insert_word(&mut self, word: &str) -> bool {
-        let word = word.to_ascii_lowercase();
+        let word = word.to_lowercase();
         if self.words.contains(&word) {
             return false;
         }
@@ -260,13 +260,13 @@ impl ConfigSettings {
 
     /// Check if a word is explicitly allowed.
     pub fn is_allowed_word(&self, word: &str) -> bool {
-        let word = word.to_ascii_lowercase();
+        let word = word.to_lowercase();
         self.words.iter().any(|w| w == &word)
     }
 
     /// Check if a word should be flagged.
     pub fn should_flag_word(&self, word: &str) -> bool {
-        let word = word.to_ascii_lowercase();
+        let word = word.to_lowercase();
         self.flag_words.iter().any(|w| w == &word)
     }
 
@@ -311,8 +311,8 @@ mod tests {
     fn test_deserialization() {
         let toml_str = r#"
         dictionaries = ["EN_US", "en_GB"]
-        words = ["CodeBook", "Rust"]
-        flag_words = ["TODO", "FIXME"]
+        words = ["CodeBook", "Rust", "Апгрейдить"]
+        flag_words = ["TODO", "FIXME", "Ошибка"]
         include_paths = ["src/**/*.rs", "lib/"]
         ignore_paths = ["**/*.md", "target/"]
         ignore_patterns = ["^```.*$", "^//.*$"]
@@ -322,8 +322,8 @@ mod tests {
         let config: ConfigSettings = toml::from_str(toml_str).unwrap();
 
         assert_eq!(config.dictionaries, vec!["en_us", "en_gb"]);
-        assert_eq!(config.words, vec!["codebook", "rust"]);
-        assert_eq!(config.flag_words, vec!["todo", "fixme"]);
+        assert_eq!(config.words, vec!["codebook", "rust", "апгрейдить"]);
+        assert_eq!(config.flag_words, vec!["todo", "fixme", "ошибка"]);
         assert_eq!(config.include_paths, vec!["src/**/*.rs", "lib/"]);
         assert_eq!(config.ignore_paths, vec!["**/*.md", "target/"]);
 
@@ -509,6 +509,20 @@ mod tests {
         assert_eq!(config, ConfigSettings::default());
     }
 
+    #[test]
+    fn test_unicode_words_ignore_case() {
+        let mut config = ConfigSettings::default();
+
+        assert!(config.insert_word("Апгрейдить"));
+        assert!(!config.insert_word("апгрейдить"));
+        assert_eq!(config.words, vec!["апгрейдить"]);
+        assert!(config.is_allowed_word("АПГРЕЙДИТЬ"));
+        assert!(config.is_allowed_word("апгрейдить"));
+
+        config.flag_words.push("ошибка".to_string());
+        assert!(config.should_flag_word("Ошибка"));
+    }
+
     #[test]
     fn test_include_tags_deserialization() {
         let toml_str = r#"
diff --git a/crates/codebook/src/dictionaries/dictionary.rs b/crates/codebook/src/dictionaries/dictionary.rs
index 3ba0d80..1f31b9d 100644
--- a/crates/codebook/src/dictionaries/dictionary.rs
+++ b/crates/codebook/src/dictionaries/dictionary.rs
@@ -137,7 +137,7 @@ pub struct TextDictionary {
 
 impl Dictionary for TextDictionary {
     fn check(&self, word: &str) -> bool {
-        let lower = word.to_ascii_lowercase();
+        let lower = word.to_lowercase();
         self.words.contains(&lower)
     }
     fn suggest(&self, _word: &str) -> Vec<String> {
@@ -150,7 +150,7 @@ impl TextDictionary {
         let words = word_list
             .lines()
             .filter(|s| !s.is_empty() && !s.starts_with('#'))
-            .map(|s| s.to_ascii_lowercase())
+            .map(|s| s.to_lowercase())
             .collect();
         Self { words }
     }
@@ -190,4 +190,14 @@ mod dictionary_tests {
         println!("{suggestions:?}");
         assert!(suggestions.contains(&"alice".to_string()));
     }
+
+    #[test]
+    fn test_text_dictionary_unicode_ignore_case() {
+        let dict = TextDictionary::new("Апгрейдить\nИИ\n");
+
+        assert!(dict.check("апгрейдить"));
+        assert!(dict.check("АПГРЕЙДИТЬ"));
+        assert!(dict.check("ии"));
+        assert!(dict.check("ИИ"));
+    }
 }