I’ve been testing the new gemini-2.5-flash-lite-preview-09-2025 model for my PDF summarization and video transcription workflows. Its faster and better than the previous version. But unfortunately, I’m encountering the exact same language bug that I reported months ago present in an early version of Gemini Flash 2.0.
The Problem
Despite providing English input and explicitly requesting English output in my prompts, the model randomly generates summaries in Spanish. What makes this particularly strange is that when this bug occurs, the output language is always Spanish - never any other language.
This makes the new preview model essentially unusable for production workflows that require consistent language output. I rely heavily on this model for processing documents and creating summaries, but the unpredictable language switching breaks my entire pipeline.
Questions
Has anyone else experienced this issue with the new September 2025 preview model?
Are there any known workarounds to force consistent English output?
Is this a known bug that’s being actively addressed?
Thank you for reaching out. To help us analyze this issue more effectively, could you please share the prompts you are using along with the full request payload? This will allow us to reproduce the issue and provide you with a more accurate solution.
I also have a specific english PDF file that triggers this Spanish/Italian output bug 100% of the time. This would be extremely helpful for your team to debug the issue.
How can I share this PDF with a Google developer? I tried reaching out to Shrestha Basu Mallick on Twitter (who asked me to DM the file), but DMs aren’t available.
Is there an official channel where I can send this test file directly to the engineering team?
Could you check if you observe the same issue with other models?
We tried to reproduce your issue, with the model you shared in provided git repo, we got response in English only and response was in English with gemini-2.5-pro as well. Although we did observed this issue with Gemini 2.5 Flash.
So just wanted to confirm with you if you observe this issue with different models and different prompts.
Thank you for testing this. I’ve just updated the repository with sample pdfs so everyone can reproduce it:
Repository Updates:
Added the problematic PDF samples directly in the sample-pdfs/ folder
Added screenshots of the bug responses in bug-screenshots/ folder
Updated the reproduction steps
Testing Results on Different Models:
I’ve now tested multiple model versions to isolate the issue:
gemini-2.5-flash-lite-preview-09-2025 (lite preview): Bug confirmed - Inconsistent language output (Italian/Spanish instead of English)
gemini-2.5-flash-preview-09-2025 (non-lite preview): No bug detected - Tested 5 generations, all responses were correctly in English
Conclusion: The bug appears to be specific to the gemini-2.5-flash-lite-preview-09-2025 model only. The regular preview version (non-lite) works correctly.
Important Note: The bug is inconsistent - you need to generate 2-3 times with the same PDF to reproduce it. Sometimes the entire response is in Italian/Spanish, sometimes it’s mixed languages, and sometimes it’s correctly in English. This inconsistency is what makes it critical for production use.
Added the problematic PDF samples directly in the 
gemini-2.5-flash-lite-preview-09-2025 (lite preview): Bug confirmed - Inconsistent language output (Italian/Spanish instead of English)