TinyMCE shows warning message when using a custom UI language

Description

1. In the web.config, set the uiCulture to a language for which Episerver does not have translations. For example:

   uiCulture="be"

2. Go to edit mode.
3. Edit an XHTML property.

Expected:
The editor loads normally.

Actual:
A warning message is shown saying that the language could not be loaded.