The text of the code is sometime highlighted and sometime not.
What does it mean and how can I turn off it?
In the comments the text is always highlighted because it’s the spell check of English and I write in an other language. How can I turn off it or change language?
Do you have a spellchecker extension installed?
This looks like you do have GitHub - jupyterlab-contrib/spellchecker: Spellchecker for JupyterLab notebook markdown cells and file editor. installed but not configured to use your language.
You need to ensure that the spellchecking language dictionary is set to the language you need.
Also, it might be that sometimes it applies spellchecking to code cells when it should not (Code cells are being spellchecked · Issue #141 · jupyterlab-contrib/spellchecker · GitHub). The workaround is to type something in the cell.
The way to un-install it would depend on how you installed it, e.g. if you did pip install jupyterlab-spellchecker
you need to run pip uninstall jupyterlab-spellchecker
I’ve found the way to change from background-box to dotted-underline but I can’t change language, it seems I can use only English.
You can change it by clicking on the language in status bar (spellchecking language is separate from the UI language which is what you were looking at in the settings menu). To add dictionaries see instructions in GitHub - jupyterlab-contrib/spellchecker: Spellchecker for JupyterLab notebook markdown cells and file editor.