Update docs around auto-selection of models

Author: lilkraftwerk

api-reference/translate.mdx

@@ -211,6 +211,10 @@ Note that we do not include examples for our client libraries in every single se
 
   When the `model_type` parameter is set, the response includes a `model_type_used` field indicating which model was used.
 
+  <Warning>
+    To ensure accurate automatic language detection, the API uses next-gen models for all requests that omit `source_lang`, regardless of `model_type`. If your source language is known, you can specify `source_lang` to allow `latency_optimized` models to be used where available for your language pair. See [supported languages](/docs/getting-started/supported-languages) for language-specific model availability.
+  </Warning>
+
   <Info>
     [Tag handling v2](/docs/xml-and-html-handling/tag-handling-v2) is compatible only with next-gen models and implicitly sets
 `model_type` to `quality_optimized`. Setting both `model_type=latency_optimized` and `tag_handling_version=v2` will return an