Hello Victor!
I have seen what you have seen before also working with translations. Have you confirmed that the lines are appearing for others in a different browser? We found that the lines were only appearing in Microsoft Edge browsers and were not appearing in Google Chrome browsers. Check that out and see if the issue persists across browsers. We used to use "BrowserStack" to test multiple devices and multiple browsers easily. I recommend that if you can.

Hello ThomasW_IronMountain
I've used "BrowserStack" as recommended and the issue persists in all browsers: Chrome, Firefox, Safari, Edge...
I have just downloaded the translation file which contains all languages:

There is html codes in this option, I guess based on format options, but can this code cause this gridlines?

Hey, for your info it seems that the root of the problem
I've just deleted the "html" codes and upload again translation file and seems it works perfectly!

Great work figuring that out! Helps to experiment.