/a - Verifies that all listed language packs can be installed or removed before processing. If any of the language packs ...

/a - Verifies that all listed language packs can be installed or removed before processing. If any of the language packs cannot be installed or removed, an error is returned and none of the specified language packs are installed or removed.

If you do not include the /a option, all specified language packs are installed or removed except the one that failed, an entry is created in the Event log, and the operation is returned as successful.
English
English (United States)
Deutsch (Deutschland)
German (Germany)
Français (France)
French (France)
日本語
Japanese
한국어
Korean
Русский
Russian