This bug may affect all field types. I've tested with JFormFieldMenutype and in a JFormFieldContent

Steps to reproduce the issue

• Set multiple languages and for the backend enable one which uses ' (Català, French or any other). I will use Català.
• Change the user language to this language.
• Go to Menú.
• Open any menú ítem (I'm using a menu item which points to a single article).
• Inspect the browser's Javascript error console and you'll find an error (see attach error1).
• On the file ca-ES/ca-ES.com_menus.ini look for the language constant (COM_MENUS_ITEM_FIELD_TYPE_LABEL) and remove the ' on the value.
• Refresh the admin screen and now the javascript error changes (see attach error2). Pls. note, that text is different.
• On the file ca-ES/ca-ES.com_content.ini look for COM_CONTENT_CHANGE_ARTICLE and remove the ' on the value.
• Refresh and no javascript errors are shown (last screenshot).

Expected result

no javascript error

Actual result

javascript error which blocks the click of the top buttons.

System information (as much as possible)

• Joomla! 3.4.4 Stable [ Ember ] 8-September-2015 21:30 GMT
• Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT
• Database version MySQL 5.6.17
• Database collation utf8_general_ci
• Català Language Pack 3.4.2.1
• PHP 5.6.10
• FireFox 40.0.3
• Windows 10

Additional comments

This bug disables the buttons on the top of the page, so is not possible to do any change on the menu items.