Use instance context in translated descriptor of Selection field

It is more expected from the translated descriptor to return translation in the
language from the instance instead of the current context.

issue5028
review15701002

CHANGELOG

@@ -1,3 +1,5 @@
+* Use instance context in translated descriptor of Selection
+
 Version 3.8.0 - 2015-11-02
 * Bug fixes (see mercurial logs for details)
 * Add test for all field methods

doc/ref/models/fields.rst

@@ -439,7 +439,8 @@ Instance methods:
 .. method:: Selection.translated([name])
 
     Returns a descriptor for the translated value of the field. The descriptor
-    must be used on the same class as the field.
+    must be used on the same class as the field. It will use the language
+    defined in the context of the instance accessed.
 
 Reference
 ---------

trytond/model/fields/selection.py

@@ -5,6 +5,7 @@ import warnings
 from sql.conditionals import Case
 
 from ... import backend
+from ...transaction import Transaction
 from .field import Field, SQLType
 
 
@@ -82,7 +83,9 @@ class TranslatedSelection(object):
     def __get__(self, inst, cls):
         if inst is None:
             return self
-        selection = dict(cls.fields_get([self.name])[self.name]['selection'])
+        with Transaction().set_context(getattr(inst, '_context', {})):
+            selection = dict(
+                cls.fields_get([self.name])[self.name]['selection'])
         value = getattr(inst, self.name)
         # None and '' are equivalent
         if value is None or value == '':