Before 2006 there was only one
language tag for the Frisian languages,
fy. 2005-11-16
ISO 639.2 identified
fy as Western Frisian, adding
frr for Northern Frisian, and
frs for Eastern Frisian. So far it is clear, ignoring the detail that Northern Frisian consists of several rather different dialects.
After the publication of
RFC 4646 the IANA language subtag registry now contains
fy,
frr, and
frs. But not
fry, this is an alpha-3 alias of
fy, and the RFC 4646 rules are roughly that shorter alpha-2 ISO 639 codes win.
It is not obvious what the
Y,
R, and
S in these subtags stand for, one plausible theory is that
S stands for
Seeltersk, the Frisian language spoken in
Saterland; see the
S marker on the map.
Interestingly ISO 639-3 uses
stq instead of
frs for this language, this is also the code used for the
stq.wikipedia. Two alpha-3 ISO 639 codes for one language are rather strange, and there is another theory to explain this situation:
frs is the code of the Lower Saxon
nds dialect spoken in Eastern Frisia, the territory including
Aurich at the
A marker on the map. In that case the
S might stand for
Saxon.
The ISO 639-2 rules make it fairly difficult to get codes for nonsense, there must be some evidence what they had in mind when frs was registered. OTOH with codes such as eur and tlh Occam's razor might miss the point.