So you might be able to do this; you probably can't make it update the subject of the conversation though, as it doesn't appear this is a functionality of the API.
First you could use the 'Search for a conversation' model to locate any conversations that are applicable, however I don't think it includes the option to search by custom attributes. But it depends how you're doing this as to whether you need it. If you can substitute your attribute for a 'tag' instead, then you could use this.
You can use the 'Reply to a conversation' model to add a note to a conversation, based on the IDs of the returned conversations and then submit a post request to apply the note.
In terms of actually automating all this though you'd either need to build an app or use something like Zapier, though I think you need the Premium plan in order to use webhooks on it.
Out of curiosity, is there a reason you want a note or subject changed if the qualification is set to EN? As if it's for the sake of clarity for your team and/or organizing different ones for different workers, you could always make a separate Team queue based on that attribute, or if it doesn't support that, have it as a tag instead.