How are the frequency pairings for localizer and glideslope decided?
As always the country is responsible for the final decision, so FAA/FCC will decide in the US (but see @Gerry 's comment from an insider below), and I believe so does the MIC in Japan. However this local decision happens after an agreement already found in international bodies. For aviation, the international body is ICAO and ICAO Chicago Convention is the main framework used to ensure international aviation operations are possible, safe and fair:
The localizer / glide path pairing table is part of Annex 10 of the Convention, Volume I, paragraph 3.1.6.
188.8.131.52 The pairing of the runway localizer and glide path transmitter frequencies of an instrument landing system shall be taken from the following list in accordance with the provisions of Volume V, Chapter 4, 4.2.
ICAO decisions in radiocommunication matters are subject to the frequency allotment and protection decisions taken under the auspices of the ITU during the World Radiocommunication Conference (WRC), held every four years. It's ITU which is responsible for the global (civil) use of the frequency spectrum.
The pairing table was built back in 1956! (Fifth Session of the COM Division, as visible on p. XI)
A preferred pairing table is provided for such use:
184.108.40.206.1 In those regions where the requirements for runway localizer and glide path transmitter frequencies of an instrument landing system do not justify more than 20 pairs, they shall be selected sequentially, as required, from the following list:
Attachment C of Annex 10, (2.6 Deployment of ILS frequencies) provides some details about this selection:
2.6.5 In general, when international use of ILS systems is confined to the pairings listed in Chapter 3, 220.127.116.11.1, the criteria are such that, provided they are met for the localizer element, the glide path element is automatically covered. At certain congested locations, where it is necessary to make assignments in both the first ten and the second ten sequence pairings, it may be necessary to select certain pairings out of sequence in order to meet the minimum geographical separation in 2.6.6.
Example: Referring to Chapter 3, 18.104.22.168.1, it will be noted that ILS Sequence Number 2 pairs the localizer frequency of 109.9 MHz with glide path frequency 333.8 MHz. Sequence Numbers 12 and 19, however, although providing wide frequency separation from Sequence Number 2 in the case of the localizers, assign frequencies of 334.1 MHz and 333.5 MHz, respectively, for the glide paths, both being first adjacent channels (300 kHz spacing) to the Sequence Number 2 glide path channel. If selection of ILS channels is confined to either the first ten or the second ten pairings, then the minimum glide path frequency separation will be 600 kHz.