In several languages' romanizations or orthographies, the letter C is used to represent the /ts/ sound. Where does this come from? Wikipedia notes that ⟨c⟩ is used for Cyrillic ⟨ц⟩ in the romanizations of Serbian and Croatian, and sometimes Ukranian. But is there a reason, or is it just that the people making the romanization system decided "well, I'm not sure what to do, so I'll just use ⟨c⟩" (which is probably why ⟨c⟩ is used for /ð/ in Fijian and for /ʕ/ in Somali). Zamenhof also used ⟨c⟩ for /ts/ in Esperanto, and used ⟨ĉ⟩ for /tʃ/. Is there any reason?
C and K both represented the /k/ phoneme in Classical Latin, but had different etymological uses: K was relegated to Greek loanwords (and occasionally kalendae), whilst C was the usual orthography (compare hiragana and katakana in modern Japanese). This separation of C, G and K in Classical Latin was consolidated in Medieval Latin and in the literacy of Western Europe. Nonetheless, K was used here and there in some vernaculars, sometimes alongside C (see e.g. the Placiti Cassinesi).
Although Classical Latin did not have a /ts/ affricate, with the rampant palatalisation that happened in Vulgar Latin, when /k/ was in front of /i/ and /e/, it "softened", undergoing velar palatalisation /kʲ/ > /c/. A similar dental palatalisation was happening with /t/. Thus by at latest the end of the 4th century CE, we see the spelling
NACIONE for the prescribed
NATIONE, even in Rome, implying this assibilation merger was well underway. As Herman (2010) reports:
Grammarians of the fifth and sixth centuries are already mentioning the [ts] pronunciation and even present it as being normal.
However, there are differences in the outcomes of /tj/ and /kj/ across the different Romance languages, which indicates the merger was far from complete when other phenomena such as intervocalic voicing intervened. Contrast the between reflexes of Latin
RATIONEM, where many modern Romance languages have voiced affricates and sibilants, and
FACIA ~ FACIEM which has voiceless reflexes across most of its descendants.
There is some evidence from non-Latin script Romance, namely Judeo-Romance: with the Hebrew script possessing tsade צ and thus able to represent a distinct /ts/ phoneme, we see strong indication of /ts/ for Roman-script French ⟨c⟩. See the following "Zarphatic" text from the late 13th century, transcribed here in a non-modernised square Hebrew transcription, in modified Brill transliteration, and in an Old French Roman 'transcription', a modern French cognate-gloss, a modern French translation, and an English translation:
פֿאצְמוֹי דְקַֿלְצֵיר דְּוַונְט ווֹש שוֹן פֵיאה דְוֵורְש דאֵיטְרְא
[p]atzᵉmoy dᵉ[q]alᵉtzeir dᵉvavnᵉt vos son pei dᵉvevrs deitrᵉ
Face moi dechalcer devant vos son pei devers deitre.
Fasse-moi déchausser devant vous son pied-devers-droite.
"Qu'il me fasse déchausser son pied droit devant vous."
"May he cause me to unshoe his right foot before you."
However, one must remember there was a competitor to the use of ⟨c⟩ during the Middle Ages, namely ⟨z⟩. In Old French, ⟨c⟩, ⟨ç⟩ and ⟨z⟩ were used together, often in the same text; this continued into the Middle French period even as /ts/ > /s/ ~ /z/. Where /ts/ continued, in what would become modern standard Italian (and also Old High German, all the way into modern standard German), it was ⟨z⟩ that would take on the orthography of /ts/. In standard Spanish orthography, ⟨c⟩ and ⟨z⟩ ended up sharing the duty of representing /ts/ > /s̪/ > /θ/.
Among the Roman script orthographies, it was Eastern European languages that both had this /ts/ phoneme in abundance, and came to a communal use of ⟨c⟩ in the age of printing. It helped that Cyrillic script Slavic orthographies possessed the difference between ⟨ц⟩ and ⟨ч~џ⟩ from Glagolitic, likely from Hebrew ⟨צ⟩ itself.
However, from the first attestations of e.g. Old Polish, we see that ⟨cz⟩ was the main way of representing all of /t͡s/, /t͡ɕ/ and /t͡ʃ/~/t͡ʂ/:
...czyebye dla, czlowyecze, dal bog przeklocz ſzobye bok, racze, nodze obye.
Ciebie dla, człowiecze, dał Bog przekłuć sobie bok, ręce, nodze obie
"For you, man, God has had his side, hands, legs pierced."
In Old Czech, in its "digraphic orthography", it was similar; occasionally a distinction was made between ⟨cz⟩ = /ts/ and ⟨czh⟩ = /tʃ/. Upon the publication of the treatise De orthographia bohemica, usually attributed to Jan Hus, in the first quarter of the 15th century, where ⟨c⟩ and ⟨ċ⟩, ⟨z⟩ and ⟨ż⟩, ⟨r⟩ and ⟨ṙ⟩ are all distinguished.
With Stanisław Zaborowski's publication of his Orthographia (1514) for Polish, we see the proscription against ⟨cz⟩, and a clarification of what ⟨c⟩ should represent:
However, one should not put ⟨z⟩ after ⟨c⟩ unless it sounds like in Polish obiczaie ‘customs’, czop ‘peg’, czas ‘time’, człoüece ‘oh man!’ and the like, where ⟨z⟩ is usually put after ⟨c⟩, but in these vulgar words ⟨c⟩ sounds rough, and there ⟨c⟩ is hard, therefore in the fashion of the other letters it can be indicated by one dot placed above it, viz. ċop, ċas, which is more regular.
However, in the vulgar words clo ‘toll’, oÿca ‘father (acc.)’, cebula ‘onion’, cuḋe ‘foreign’ and the like, ⟨c⟩ must not be followed by ⟨z⟩, nor must it be marked with any dot, for here it is used with its proper meaning.
It is likely from this Pan-Slavic use of ⟨c⟩ = /ts/ that Esperanto gained its use, and also likely how /tsʰ/ came about in Mandarin Chinese Pinyin. However, there are also many languages that use ⟨c⟩ to represent /t͡ʃ/ or /d͡ʒ/; most of these gained their orthographies in the 19th or 20th centuries, often under influence from speakers of Western European languages.
This is true for the scripts of languages like Xhosa, Zulu, Somali and Fijian, where the very redundancy of the ⟨c⟩ glyph amongst European languages means that it can take whatever value language planners prefer. It is possible that the 1971 Somali Latin script used ⟨c⟩ because of its visual similarity to the Arabic ﻉ ʿayn (although in much more modern times, with the rise of the Arabic chat alphabet, ﻉ has a new consensus 'romanisation': 3).
This is also definitely the case for Fijian, for which we have the testimony of the script designer, Welsh missionary David Cargill, and specifically the diary of his printer John Hobbs:
I next printed a leaflet for Fiji. Mr. Cargill said, “I want you to cast me some Greek thetas.” I said, “The Th in Fijian is flat [i.e. voiced], and I am not a type founder; take one of our spare letters and make that do.” In a short time I got the thing printed giving C the sound of Th.