When importing an OPML, the text attribute (which is compulsory) seems to be ignored and title (which is optional and often misused¹) is taken into account.
This means that when importing a valid OPML with tags/groups like this :

<outline title="feed1" text="feed1" .../>
<outline title="feed2" text="feed2" .../>

the tag "mytag is not created".
To be created, it needs a title (but shouldn't²) :

In particular selfoss fails to import properly kriss feed's opml.
For compatibility, since it seems to be a common mistake, I propose to keep looking at the title attribute for the tag name, but also to the text attribute if title isn't present.

¹ http://www.therssweblog.com/?guid=20051003145153
² AFAIK, I might have misunderstood the specification