How do I define a computed field equal to the concatenation of two fields of my custom entity?
I found this in ContentEntityBase.php 499 (but you could show us the whole error log):

I think the case is the following: your field is translatable (you didn't set it's not, and I tihnk the default is TRUE).  

else {
  $field_langcode = $langcode;
}
**$field-&gt;setLangcode($field_langcode);**
$this-&gt;fields[$name][$langcode] = $field;


What I don't understand here: how basefield definition allows you to add a class which doesn't inherit / implements the right class / interface. I think it's a core bug?

You should either:
- add set language method, with a languagecode property
- set your computed field untranslatable