Instances with I18N internationalization plugins fail to apply the type-ahead auto-completer when switched to a non-English language.

Analysis of the generated html across English / French show the following differences:

...if (!e.ac) new AJAXTableCompleter(gel('sys_display.incident.watch_list')

...if (!e.ac) new AJAXReferenceCompleter(gel('sys_display.incident.watch_list')

1. Login any base system instance on any release in English language.
2. Navigate to Plugins and activate the French plugin.
   Observe the dictionary attributes for field task.watch_list
   slushbucket_ref_no_expand=true,no_sort=true,ref_ac_columns=first_name
3. Open an incident form.
4. Unlock the Watch List
5. Type 't' for names search.
   Note the resulting rows displayed.
6. Switch to French language.
7. Repeat steps 3-5.
   Note the different results.

After carefully considering the severity and frequency of the issue, and the risk of attempting a fix, it has been decided to not address this issue in any current or future releases. We do not make this decision lightly and we apologize for any inconvenience. If you have any questions regarding this problem, please contact ServiceNow Technical Support.

However, limited to reference fields only, you can work around the behaviour by explicitly forcing the field dictionary attribute with:

ref_auto_completer=AJAXTableCompleter

So for example, in case of the Watch List:

ref_auto_completer=AJAXTableCompleter,slushbucket_ref_no_expand=true,no_sort=true,ref_ac_columns=first_name