[Tex2002ans]

Originally Posted by bookman156

So you'd add a class to that to get your words italic. In what contexts does declaring the spanned phrase as French help?

Proper HTML lang markup helps with:

• Dictionary
  
  • Highlighting the French word, a device may pop up a French->English dictionary instead.
    
• Translation
  
  • French->English translate may occur.
    
• Text-to-Speech (TTS)
  
  • Can speak in the correct accent/model.
    
• Hyphenation
  
  • Different languages have completely different hyphenation rules.
    
• Spellchecking
  
  • Red squigglies.
    

Current, real-life use-cases?

• Dictionary
  
  • Press-and-hold on a French word, get the French->English dictionary.
    
  • Press-and-hold on an English word, get the English dictionary.
    
• Translation
  
  • EPUB readers, such as PocketBook, have in-line translation.
    
    • Similar to Google Translate on a website, PocketBook can translate and substitute, on-the-fly, the converted-to-English words.
      
• Text-to-Speech
  
  • EPUB readers such as Thorium are able to take advantage of this. (Along with Screen Readers.)
    
    • French will be spoken by French TTS, English will be spoken by English TTS.
      
• Hyphenation
  
  • While most programs only handle language at the document-/book-level, Kindles (KFX format) can currently go down to per-word.
    
  • LaTeX/Word/LibreOffice can also take full advantage of this.
    
• Spellchecking
  
  • Way less red squigglies!
    
  • If you Right-Click a misspelled word, you may get more accurate recommendations.
    
  • Sigil/Calibre have the fantastic Spellcheck Lists. (Sortable, searchable lists of all words in the book + their language.)
    
  • Word/LibreOffice can also take advantage of this.
    

I've written about each of these things extensively. Most recently, see 2021: "Add xml:lang to ePub".

There are always new tools/updates happening. Things have gotten MUCH better on each of these fronts, even within the past few years.

(For example, because of my consistent nudging since 2016, Multi-Language Spellchecking is now a thing in both Sigil/Calibre.)

If you marked your HTML languages properly, boom, you automatically benefit from each of these enhancements as they come along.