<p>How do I add the Swedish interactive user,  </p>

<pre><code>NT INSTANS\INTERAKTIV  
</code></pre>

<p>or the English interactive user,  </p>

<pre><code>NT AUTHORITY\INTERACTIVE  
</code></pre>

<p>or any other localised user group with <strong>write</strong> permissions to a program folder's ACL?</p>

<p>Is this question actually "How do I use <strong>secureObject</strong>"? I cannot use the <strong>LockPermissions Table</strong> because I undestand inheritance is removed. <strong>secureObject</strong> permissions seem to require <strong>CreateDirectory</strong> rather than <strong>Directory</strong>... </p>

There is no way
as such
to add both account names to an ACL since they are one and the same. The name you see corresponds to a SID, and that SID is identical in both the English and Swedish localizations. In the case of the INTERACTIVE group, that SID is
```
S-1-5-4
```
.
I haven't followed WiX in a long while, but I expect there has to be a way to specify SIDs for ACLs instead of account names. You should never, ever rely on the account name for well-known accounts unless there is absolutely no way to avoid it. Here is a
list of well-known SIDs
for reference.
Edit:
This post
seems to provide a solution to your problem using a custom action to translate the SIDs to account names - apparently WiX doesn't out of the box support using SIDs for Permission or PermissionEx objects.
Here is a more authoritative list of well-known SIDs in
Q243330
of the Microsoft Knownledge Base.