diff --git a/src/highlighters.cc b/src/highlighters.cc
index 52db836d..bcab234e 100644
--- a/src/highlighters.cc
+++ b/src/highlighters.cc
@@ -884,7 +884,7 @@ void expand_unprintable(const Context& context, HighlightFlags flags, DisplayBuf
                 {
                     auto coord = it.coord();
                     Codepoint cp = utf8::read_codepoint<utf8::InvalidPolicy::Pass>(it, end);
-                    if (cp != '\n' and not iswprint(cp))
+                    if (cp != '\n' and not std::isprint((wchar_t)cp, std::locale{}))
                     {
                         if (coord != atom_it->begin())
                             atom_it = ++line.split(atom_it, coord);
diff --git a/src/ranked_match.cc b/src/ranked_match.cc
index 89eb2c85..6aa89e52 100644
--- a/src/ranked_match.cc
+++ b/src/ranked_match.cc
@@ -34,6 +34,7 @@ using Utf8It = utf8::iterator<const char*>;
 
 static int count_word_boundaries_match(StringView candidate, StringView query)
 {
+    std::locale locale;
     int count = 0;
     Utf8It query_it{query.begin(), query};
     Codepoint prev = 0;
@@ -41,8 +42,10 @@ static int count_word_boundaries_match(StringView candidate, StringView query)
     {
         const Codepoint c = *it;
         const bool is_word_boundary = prev == 0 or
-                                      (!iswalnum(prev) and iswalnum(c)) or
-                                      (islower(prev) and isupper(c));
+                                      (!std::isalnum((wchar_t)prev, locale) and
+                                       std::isalnum((wchar_t)c, locale)) or
+                                      (std::islower((wchar_t)prev, locale) and
+                                       std::isupper((wchar_t)c, locale));
         prev = c;
 
         if (not is_word_boundary)
diff --git a/src/unicode.hh b/src/unicode.hh
index 9b65e265..83c63fa1 100644
--- a/src/unicode.hh
+++ b/src/unicode.hh
@@ -2,6 +2,7 @@
 #define unicode_hh_INCLUDED
 
 #include <wctype.h>
+#include <locale>
 
 namespace Kakoune
 {
@@ -28,7 +29,7 @@ enum WordType { Word, WORD };
 template<WordType word_type = Word>
 inline bool is_word(Codepoint c)
 {
-    return c == '_' or iswalnum(c);
+    return c == '_' or std::isalnum((wchar_t)c, std::locale{});
 }
 
 template<>
@@ -67,8 +68,8 @@ inline CharCategories categorize(Codepoint c)
     return CharCategories::Punctuation;
 }
 
-inline Codepoint to_lower(Codepoint cp) { return towlower((wchar_t)cp); }
-inline Codepoint to_upper(Codepoint cp) { return towupper((wchar_t)cp); }
+inline Codepoint to_lower(Codepoint cp) { return std::tolower((wchar_t)cp, std::locale{}); }
+inline Codepoint to_upper(Codepoint cp) { return std::toupper((wchar_t)cp, std::locale{}); }
 
 inline char to_lower(char c) { return c >= 'A' and c <= 'Z' ? c - 'A' + 'a' : c; }
 inline char to_upper(char c) { return c >= 'a' and c <= 'z' ? c - 'a' + 'A' : c; }