Thank you Akhil. I hadn't imported the library in the Python code. I 've just done it and add the _(" ") to some labels, but still same result. It's weird because I did several reports and I've never needed to write _(" ") around the words, they were automatically recognized by the translation tool.

In reports you don't need to use "_()". if some field strings, warnings or some texts, you need to translate, it should be used I think. In reports, like you told, it detects automatically.

Finally, I created the translation file manually and it works (I was able to this because there were only a few terms to translate). However, even doing the translation file by myself, a term isn't being translated, the sentence just after the OR: [[ (o.comment and format(o.comment)) or "Please, indicate the invoice number on the concept of income" ]] Which could be the problem?