diff --git a/src/ocrmypdf/api.py b/src/ocrmypdf/api.py
index b90973e9..605b8b1c 100644
--- a/src/ocrmypdf/api.py
+++ b/src/ocrmypdf/api.py
@@ -283,6 +283,34 @@ def _check_no_conflicting_ocr_params(
         )
 
 
+def _remap_language_to_languages(options_kwargs: dict) -> None:
+    """Map the public API 'language' parameter to OcrOptions 'languages' field.
+
+    The public API uses 'language' (matching CLI --language) but OcrOptions
+    uses 'languages' (plural). This also coerces a bare string to a list
+    and splits '+'-separated language codes (e.g. 'eng+deu' -> ['eng', 'deu'])
+    to match the CLI behavior.
+    """
+    if 'language' in options_kwargs and 'languages' not in options_kwargs:
+        lang = options_kwargs.pop('language')
+        if lang is None:
+            return
+        if isinstance(lang, str):
+            lang = lang.split('+')
+        else:
+            # Flatten any '+'-separated entries in the list
+            expanded: list[str] = []
+            for item in lang:
+                if isinstance(item, str) and '+' in item:
+                    expanded.extend(item.split('+'))
+                else:
+                    expanded.append(item)
+            lang = expanded
+        options_kwargs['languages'] = lang
+    elif 'language' in options_kwargs:
+        del options_kwargs['language']
+
+
 def create_options(
     *, input_file: PathOrIO, output_file: PathOrIO, parser: ArgumentParser, **kwargs
 ) -> OcrOptions:
@@ -304,6 +332,9 @@ def create_options(
     # Prepare kwargs for direct OcrOptions construction
     options_kwargs = kwargs.copy()
 
+    # Map API parameter 'language' to OcrOptions field 'languages'
+    _remap_language_to_languages(options_kwargs)
+
     # Set input and output files
     options_kwargs['input_file'] = input_file
     options_kwargs['output_file'] = output_file
@@ -762,6 +793,9 @@ def _pdf_to_hocr(  # noqa: D417
         ):
             options_kwargs[param_name] = param_value
 
+    # Map API parameter 'language' to OcrOptions field 'languages'
+    _remap_language_to_languages(options_kwargs)
+
     # Handle plugins
     if plugins:
         options_kwargs['plugins'] = plugins
diff --git a/tests/test_api.py b/tests/test_api.py
index dc32fdfb..03db01db 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -22,6 +22,64 @@ def test_language_list():
         ocrmypdf.ocr('doesnotexist.pdf', '_.pdf', language=['eng', 'deu'])
 
 
+def test_language_parameter_mapped_to_languages():
+    """Test that the API 'language' parameter is mapped to OcrOptions 'languages'.
+
+    Regression test for GitHub issue #1640: the Python API ignored the language
+    parameter, always defaulting to 'eng'.
+    """
+    from ocrmypdf._options import OcrOptions
+    from ocrmypdf.api import create_options, setup_plugin_infrastructure
+    from ocrmypdf.cli import get_parser
+
+    setup_plugin_infrastructure()
+    parser = get_parser()
+
+    options = create_options(
+        input_file='test.pdf',
+        output_file='output.pdf',
+        parser=parser,
+        language=['tam'],
+    )
+    assert options.languages == ['tam']
+
+    # Test with a list of multiple languages
+    options = create_options(
+        input_file='test.pdf',
+        output_file='output.pdf',
+        parser=parser,
+        language=['fra', 'deu'],
+    )
+    assert options.languages == ['fra', 'deu']
+
+    # Test with a bare string (single language)
+    options = create_options(
+        input_file='test.pdf',
+        output_file='output.pdf',
+        parser=parser,
+        language='tam',
+    )
+    assert options.languages == ['tam']
+
+    # Test '+'-separated string is split like CLI --language
+    options = create_options(
+        input_file='test.pdf',
+        output_file='output.pdf',
+        parser=parser,
+        language='eng+spa',
+    )
+    assert options.languages == ['eng', 'spa']
+
+    # Test '+'-separated entry within a list is also split
+    options = create_options(
+        input_file='test.pdf',
+        output_file='output.pdf',
+        parser=parser,
+        language=['eng+spa'],
+    )
+    assert options.languages == ['eng', 'spa']
+
+
 def test_stream_api(resources: Path):
     in_ = (resources / 'graph.pdf').open('rb')
     out = BytesIO()