diff --git a/src/harmony/matching/matcher.py b/src/harmony/matching/matcher.py
index 46a59db..ac39444 100644
--- a/src/harmony/matching/matcher.py
+++ b/src/harmony/matching/matcher.py
@@ -115,7 +115,22 @@ def add_text_to_vec(text, texts_cached_vectors, text_vectors, is_negated_, is_qu
     return text_vectors
 
 
-def process_questions(questions: list, texts_cached_vectors: dict, is_negate: bool) -> list[TextVector]:
+def get_question_languages(instruments: List[Instrument]) -> dict:
+    """
+    Map each question text to the language of the instrument it came from, so that negation uses the right rules.
+    """
+    question_languages = {}
+    for instrument in instruments:
+        language = getattr(instrument.language, "value", instrument.language) or "en"
+        for question in instrument.questions:
+            question_languages[question.question_text] = language
+    return question_languages
+
+
+def process_questions(questions: list, texts_cached_vectors: dict, is_negate: bool,
+                      question_languages: Optional[dict] = None) -> list[TextVector]:
+    if question_languages is None:
+        question_languages = {}
     text_vectors: List[TextVector] = []
     for question_text in questions:
         # Skip None or whitespace-only texts
@@ -134,7 +149,7 @@ def process_questions(questions: list, texts_cached_vectors: dict, is_negate: bo
         text_vectors = add_text_to_vec(question_text, texts_cached_vectors, text_vectors, False, False)
 
         if is_negate:
-            negated_text = negate(question_text, 'en')
+            negated_text = negate(question_text, question_languages.get(question_text, "en"))
         else:
             negated_text = question_text
         text_vectors = add_text_to_vec(negated_text, texts_cached_vectors, text_vectors, True, False)
@@ -173,14 +188,16 @@ def create_full_text_vectors(
         query: Optional[str],
         vectorisation_function: Callable,
         texts_cached_vectors: dict[str, list[float]],
-        is_negate: bool
+        is_negate: bool,
+        question_languages: Optional[dict] = None
 ) -> tuple[List[TextVector], dict]:
     """
     Create full text vectors.
     """
 
     # Create a list of text vectors
-    text_vectors = process_questions(all_questions, texts_cached_vectors, is_negate=is_negate)
+    text_vectors = process_questions(all_questions, texts_cached_vectors, is_negate=is_negate,
+                                     question_languages=question_languages)
 
     # Add query
     if query:
@@ -236,7 +253,8 @@ def match_instruments_with_catalogue_instruments(
         query=None,
         vectorisation_function=vectorisation_function,
         texts_cached_vectors=texts_cached_vectors,
-        is_negate=is_negate
+        is_negate=is_negate,
+        question_languages=get_question_languages(instruments)
     )
 
     # For each instrument, find the best instrument matches for it in the catalogue
@@ -626,7 +644,8 @@ def match_instruments_with_function(
         query=query,
         vectorisation_function=vectorisation_function,
         texts_cached_vectors=texts_cached_vectors,
-        is_negate=is_negate
+        is_negate=is_negate,
+        question_languages=get_question_languages(instruments)
     )
 
     # get vectors for all original texts and vectors for negated texts
diff --git a/tests/test_negation_language.py b/tests/test_negation_language.py
new file mode 100644
index 0000000..5c37bf3
--- /dev/null
+++ b/tests/test_negation_language.py
@@ -0,0 +1,68 @@
+'''
+MIT License
+
+Copyright (c) 2023 Ulster University (https://www.ulster.ac.uk).
+Project: Harmony (https://harmonydata.ac.uk)
+Maintainer: Thomas Wood (https://fastdatascience.com)
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+'''
+
+import sys
+import unittest
+
+sys.path.append("../src")
+
+from harmony.matching.matcher import get_question_languages, process_questions
+from harmony.schemas.requests.text import Instrument, Question
+
+
+class TestNegationLanguage(unittest.TestCase):
+    """Regression tests for issue #142: negation must use the instrument's language, not always English."""
+
+    def setUp(self):
+        self.instrument_en = Instrument(language="en", questions=[Question(question_text="I feel depressed")])
+        self.instrument_pt = Instrument(language="pt", questions=[Question(question_text="eu me sinto deprimido")])
+
+    def get_negated_texts(self, instruments):
+        question_languages = get_question_languages(instruments)
+        all_questions = [q.question_text for i in instruments for q in i.questions]
+        text_vectors = process_questions(all_questions, {}, is_negate=True, question_languages=question_languages)
+        return [tv.text for tv in text_vectors if tv.is_negated]
+
+    def test_question_languages_follow_instrument(self):
+        question_languages = get_question_languages([self.instrument_en, self.instrument_pt])
+        self.assertEqual("en", question_languages["I feel depressed"])
+        self.assertEqual("pt", question_languages["eu me sinto deprimido"])
+
+    def test_portuguese_item_uses_portuguese_negation(self):
+        negated = self.get_negated_texts([self.instrument_en, self.instrument_pt])
+        self.assertIn("never I feel depressed", negated)
+        self.assertIn("não eu me sinto deprimido", negated)
+        self.assertNotIn("never eu me sinto deprimido", negated)
+
+    def test_default_is_english(self):
+        text_vectors = process_questions(["I feel depressed"], {}, is_negate=True)
+        negated = [tv.text for tv in text_vectors if tv.is_negated]
+        self.assertEqual(["never I feel depressed"], negated)
+
+
+if __name__ == '__main__':
+    unittest.main()