XmlWriter class has a localization issue.
While decimal format used for writing coordinates is fixed and uses decimal point, the bounds object is written just using String.format with "%f" flag, without providing explicit locale (and thus using a system default one).

In Russian (and some others) locales by default a comma is used as decimal separator. So once I save an OSM file I can not read it anymore as it has inconsistent delimiters: all coordinates use point but bounds use comma.

Please specify Locale.US (or any other locale with decimal point) explicitly in XmlWriter.templateBounds field to force it use consistent delimiters everywhere in a generated file.

By the way, it will also be nice to make OsmOutputStream implement AutoClosable to be able to work with try-with-resource blocks.