Screen readers use the site’s declared language to figure out what pronunciation rules to use when reading the content. If no language is declared, then the screen reader will generally use the screen reader user’s installation’s language or try to guess: e.g., if a French user of a screen reader visits your page, and no HTML language is declared, it will either read as if the text was written in French, or attempt to guess the actual language and pronunciation. Declaring the default language greatly improves comprehension of the text.
As problems go, this is relatively rare as a problem in a theme: what is actually a bigger problem is sections of text that are not in the primary language of the site. This plug-in doesn’t currently handle changing languages in the middle of content. Further reading about document language.