englyn ~ englynion: A Welsh poem, I think. Similarly pennill ~ pennillion, but eisteddfod ~ eisteddfodau, geneth 'girl' ~ genethod. Welsh has a variety of plural endings with no real predictability, plus it uses umlaut (bachgen 'boy' ~ bechgyn) and some mixed plurals.
ruba`iyyah ~ ruba`iyyat. Arabic feminine weak plural. The feminine singular -ah (often pronounced & transcribed -a as in Fatimah or Fatima) becomes long stressed -at in the plural. Ruba`iyyah is from the root for 'four' ('arba`) and means 'quatrain' (< quatre). The -ah ending is usually feminine but in this case it has another miscellaneous noun-formative function; it still takes the plural -at. The plain or masculine it's attached to could be transcribed ruba`iyy or ruba`iy or ruba`i - the i is long and stressed, and the yy is to some extent a spelling convention of Arabic script. I'm not sure that you can go from masculine singular ruba`i(yy) to feminine plural ruba`iyyat: this looks like an error to me, but I don't know.
pruta(h) ~ prutot(h). The Hebrew equivalent of the Arabic feminine plural.
kibbutz ~ kibbutzim, so also cherub~im etc. Hebrew masculine plural. The Arabic equivalent (not in your list) is nominative -un, accusative and genitive -in, usually imported into English as -in, e.g. fellah 'peasant' ~ fellahin. From khams '5' comes khamsun, khamsin '50', and the wind is so named (I think) because it blows for fifty days.
falaj ~ aflaj. I don't know what this word means but it's an Arabic "broken" plural. Many (most) masculine words change to a different vowel pattern in the plural, rather than taking the -un/-in ending. There are numerous common patterns but it's pretty unpredictable which will be used. kitab 'book' ~ kutub; walad 'son' ~ 'awlad; su'al 'answer' ~ 'as'ilah; sometimes weak and broken plurals are both used, e.g. talib 'student' ~ talibun or tullab. (Borrowed into Persian as taleb, with Persian plural taleban.)
vila ~ vily is a Slavonic language, but I can't say which: Czech? Bulgarian? Alluring ghostly maidens, most familiar as the Wilis in Giselle, the ghosts of brides who were jilted before their wedding. Vily appear in the fourth Harry Potter book as cheerleaders for the Bulgarian quidditch team.
paries ~ parieties still looks like wrong Latin: shouldn't it be parietes?
mganga ~ waganga Swahili 'wizard, native doctor', I think. Names of persons go in the m- class in Swahili and take the plural wa-, e.g. also mtu 'person' ~ watu. Other Bantu languages use similar prefixes, often ba- for the plural, as in Bantu itself, 'people' = watu. One person from Lesotho is a Mosotho, a plurality of them are Basotho. Borrowings into English from any Bantu are very rare, so we don't see these plurals much. The curency of Swaziland is the lilangeni, plural emalangeni.
