This is sets language. I need to just return name of chosen language.

Actually it does not work like that. As lang field is not directly related with res.lang object. If you looked in code, you would see that lang field is selection field which uses function, which returns code of language chosen.

Yes you are right, I updated my answer with something that'll help you achieve what you need

I added in 2 possiblities.

This is a bit hard coded, but looks like it's the only way now. Thanks.

If you want to avoid hard coding, you could change the field type of the lang column to be a many2one and have a reference to the language object, that way you can just use o.partner_id.lang.name

Yes it's possible, but if this field would be changed, I imagine all other reports that use this field, would stop working properly as they all would need to be modified.

Yes indeed. Let's see, how about you add a new hidden many2one field that you update with a method mapped to the on_change attribute of the lang field, where you could fill the new field with the language object reference depending on the choice of the user and use it directly in your report.