Currently we manually localize and update the DOM-elements of the AltText-button, and it seems nicer to utilize Fluent "properly" for that task.
This can be achieved by introducing an explicit `span`-element on the AltText-button (similar to e.g. the regular toolbar-buttons), and adding a few more l10n-strings, since that allows just setting the `data-l10n-id`-attribute on all the relevant DOM-elements.
Finally, note how we no longer need to localize any strings eagerly when initializing the various editors.
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