Is your feature request related to a problem? Please describe.

"Nemeth" is unsuitable as a default Braille output code, because:

• Although Unified English Braille is the standard globally for English texts, mathematics codes in the English-speaking world have evolved very differently in different places. A vocal minority of North American Braille users prefer the Nemeth code instead of UEB due to its compactness compared to other systems (especially important on refreshable Braille displays with limited real estate) and/or for traditional or philosophical reasons. Other users find the Nemeth number representation system and modal nature confusing and prefer a system that represents symbols used in mathematical contexts in the same manner as they are represented in other contexts.
• For non-English languages (for instance, various mainland European languages), Nemeth symbols conflict with numbers or other punctuation symbols, making the reading experience potentially misleading.
• Nemeth notation is relatively unknown outside Canada and the US, and is likely to be especially unknown in developing countries.

Describe the solution you'd like

Use ASCIIMath as a sensible global default (easy, rejected in #19232), or add infrastructure to set a default Braille math output code per language (better for languages with an uncontroversial sensible default). In the latter approach, we might start by mapping fi -> ASCIIMath-finnish, es -> CMU, vi -> Vietnamese, and sv -> Sweedish, and set ASCIIMath as a fallback for languages where a sensible default cannot be determined. I personally think en -> UEB is reasonable, but know it's contentious in some circles.

Describe alternatives you've considered

Leave Nemeth as a default globally (I don't consider this reasonable, see above).