@primarymay it's tell me "unable to create comment" so I'm having to cut things down.
The labor contracts act article 18 specifies the terms for permanent conversion (
https://www.japaneselawtranslation.go.jp/en/laws/view/3744/en)
BUT the employers you're talking about are universities presumably.
Ostensibly because this completely screws with tenure-track, MHLW and MEXT came up with an exception (
https://www.mext.go.jp/a\_menu/koutou/shinkou/1410626.htm) that applies to universities and certain research institutions. so "researchersなど" and people with jobs under the tenure track system.
Some universities decided language teachers don't fit; others decided they do. @nicoleb07 for instance was at Tohoku university which decided they do. Conversely where I work full-time, I used the law at five years under the advice of a lawyer who said they wouldn't be able to convincingly show that I was a researcher.
Thus, the discrepancy on the employer side.
Moving to the legal side, a case in Tokyo [東京地判令和3年12月16日(第1審)労働判例1259号41頁] involving Sensyuu daigaku and a part-time language teacher resolved that they had the right to permanent conversion at 5 (
https://note.com/mizonobeyuki/n/nbf11a1d0a059). The most relevant part of the holding: そのため、
研究を行わず教育のみを担当する講師についてはここでいう「研究者」に該当しない。 (For that reason, someone who is employed only for education and not research does not fall under the definition of "researcher")
Conversely there are older examples of people being told to pound sand and wait 10 years by courts (
https://www.tis.amano.co.jp/hr\_news/3194/)
on a slightly different note, here's a lawyer explaining a tohoku university accusing the university of article 19 雇い止め as not violating article 19 (
https://hk-plazalaw.com/column/hanrei024). Similar outcomes for Article 19 abound (doesn't rise to the level of being rational). Conversely, a Belgian managed to sue Nagasaki University and get permanent employment even though he signed a 3-year and then 2-year contract (
https://mainichi.jp/english/articles/20230915/p2a/00m/0na/038000c).
switching to part-timers, I've asked lots of the people here and none of them really want to rock the boat on this. My sense is that it's easier to still find a way to not find a class for a part-timer if they converted and the university was angry.
Conversely, no one here full-time is motivated to lose the part-timers we already have and wind up with a system where we have to get rid of them or gap them for more than 6 months.
all of that to say, it's a bit of a mess and if you are really concerned talk to a lawyer.
My interpretation (not a lawyer) is that they're on weak ground applying the same 10-year exception to language teachers precisely for the reason's spelled out in the senshuu university judgment. But the only way to figure out the outcome is to give them the permanent conversion document after 5 years and see what they do.
Merely writing "the 10-year exception applies" does not make it so because contract terms can't give you worse terms than the law or constitution or employment rules.