The lang attribute specifies the primary language used in contents and attributes containing text content of particular elements.
There is also an xml:lang attribute (with namespace). If both of them are defined, the one with namespace is used and the one without is ignored.
In SVG 1.1 there was a lang attribute defined with a different meaning and only applying to <glyph> elements. That attribute specified a list of languages according to RFC 5646: Tags for Identifying Languages (also known as BCP 47). The glyph was meant to be used if the xml:lang attribute exactly matched one of the languages given in the value of this parameter, or if the xml:lang attribute exactly equaled a prefix of one of the languages given in the value of this parameter such that the first tag character following the prefix was "-".
You can use this attribute with any SVG element.