Some languages use comma (,) instead of point (.) to denote decimals (and viceversa with the thousands punctuation). But the math renderer only renders correctly the point and creates a small space next to the comma, that shouldn't exist.
You can test it in Basque Wikipedia, for example, easily.
This behavior matches TeX/LaTeX. Wrapping {} around the comma will remove the extra space, e.g., $12{,}34$.
Since the database of equation rendering is currently shared across WMF sites, there's currently no way to automate such localization features.
@Theklan: We are currently discussing some changes to the math extension here: T195861, you are welcome to discuss this issue. Currently the plan is to
As you can see this is a tremendous amount of work, progress is quite slow and everything relies on volunteers. I don't think changing the syntax for existing formulas with commas is feasible and maintaining the current system without additional localization features already overstretches our resources. I think it would be a better idea to just keep using the well-known {,} for decimal comma and make sure that help pages mention this LaTeX hack.
It might be worth documenting the workaround in Help:Formula for the affected languages.
I think the way to go is to solve the problem on the input side. We are currently working on suggestions for the visual editor. I think suggesting to replace , with {,} in certain cases would be the way to go. Otherwise, people expecting the TeX , rendering might be upset.