Please add this PR to Accessibility category on issues.joomla.org.

Done

I have tested this item ✅ successfully on 93a9ed7

@brianteeman is there a particular reason that you're hardcoding the tab index instead of setting it to 0 (auto)?

Re: https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/tabindex

I was following the existing code which I assumed was there for a reason

I have tested this item ? unsuccessfully on 93a9ed7

Set the tabindex for all elements = 0.
If the tabindex > 0, the tab order is incorrect.
Correct tab order: Username, the tip for Username, Password, the tip for Password, etc.
Incorrect tab order: Username, Password, Language, Login, the tip for Username, the tip for Password, etc.

In addition, in order to solve the problem of accessibility comprehensively, it would be necessary to add the missing legend tag for a fieldset tag (<legend>Form login</legend>).

@dgt41 I remember now why it doesnt have tabindex=0 it is so that keyboard users do not have to tab past the useless tooltip. So it is one tab to move to the next field and not two.

@wojsmol This PR fixes the order of the tabs so that language is in the correct place. I prefer to contribute to joomla so that we see real improvements for our users today. I will remove the words a11y from the title and then you cannot say it is no good because it fixes a bug, If the a11y team ever start to work then they can contribute a fully accessible and usable fix - I would rather not wait

@brianteeman : I must admit that I completely do not understand your comment.

1. You have marked your PR with the a11y label, not @wojsmol. @wojsmol only asked for a label to be added to Tracker Issue. Has he behaved improperly? I do not think so.
2. If the code doesn't really have tabindex=0 so that keyboard users don't have to put it into useless toltips, then this explanation shows the foolishness of what they did. And sooner this stupidity needs to be repaired.

3. You wrote that:

I will remove the words a11y from the title and then you cannot say it is no good because it fixes a bug.

No, unfortunately. Does not fix the error. On the contrary, it preserves it and creates the appearance that it is good.
4. If it's too difficult for you, I'll make the PR (I'm from the accessibility team, as you probably know). However, I thought that if you had already discovered this problem, it would not make it difficult for you to remove unnecessary tabindexes and possibly add one line of code with a new language sequence (legend).
However, I think you can do it yourself - I don't want to take away your authorship.
All tabindex attributes in this code need to be removed - they are unnecessary (so you don't need to set them to 0), because form controls automatically receive a focus according to the source ordering.

I have already explained

I have tested this item ✅ successfully on 93a9ed7

RTC after two successful tests.

Thanks