Braille grade indicator on language attribute

## Consider using lang attribute to indicate the braille grade.

The lang attribute should be included in all XHTML and HTML documents. It can also be placed on other elements to indicate a change in the language. There are also provisions for using subtags and mechanism for extensions.

We should consider this approach as an alternative to using class attributes on span and div as described in issue number: 142

### Language tag syntax
[Language tag syntax](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/lang#content)

Script subtag
Optional. This subtag defines the writing system used for the language, and is always 4 characters long, with the first letter capitalized. For example, French-in-Braille is fr-

### Specification for language tags

[ Tags for Identifying Languages](https://datatracker.ietf.org/doc/html/rfc5646))
This document describes the structure, content, construction, and
semantics of language tags for use in cases where it is desirable to
indicate the language used in an information object. It also
describes how to register values for use in language tags and the
creation of user-defined extensions for private interchange.

Start by reading issue 142, then review the linked MDN language-attribute guidance and RFC 5646. Determine whether language tags can represent the braille grade use case and document the chosen approach and required scope; the issue does not name files or tests.

Written by the indexing model from the issue text.