В файле конфигурации NAV службы (CustomSettings.config) есть узел.
Specifies which of the installed Dynamics NAV languages on the server instance will be used as the default language in the clients. Set the value to a valid language culture name, such en-US or da-DK.
In the Dynamics NAV Web and Tablet clients, the Default Language setting determines the language that is used if the web browser's language setting does not match any installed language or a language in the Supported Languages setting, if used. In the Dynamics NAV Windows client, this is the language that is used if the language setting of the computer does not have a match.
If there are application-specific configuration settings, this setting will be overridden by the default language setting that is specified in application-specific configuration file.
<add key="DefaultLanguage" value="" />