[XML] Remove string attrribute from views. There is a script on https://bugs.tryton.org/msg26170 to replace them.
[SQL] Language codes have been simplified. If you want to keep custom translation you must update translation code to match the new code.
Here is an example for the code change from
UPDATE ir_translation SET lang = 'en' WHERE lang = 'en_US';
ir_configuration may also contain language code:
UPDATE ir_configuration SET language = ‘en’ WHERE language = ‘en_US’;
You could also rename existing po file to match the new code.