Most languages have some sort of register, unless you're communist. Javanese is pretty egregious in this regard because the politness is conveyed by suppletion.



In Javanese case, the suppletions can come from loan words. Like for example:
- jeneng < Native
- nami < Sanskrit
- asma < Arabic

Tell me about register in your language.

Oksirapho has a painfully Byzantine register system that I am still vaguely drafting. So far I'm pretty firm on the idea of a large number of alternative verbs (in addition to a ton of honourifics) and possibly adverbs. The usage of these would vary depending on the speaker and audience, with probably four or five different registers total, albeit overlapping irregularly and rarely having verbs that supplete unique alternatives for all of the registers.

Lilitika was self-consciously designed to avoid register and honourifics as much as possible (for, honestly, fairly communist reasons) but it gradually adopted a formal/ceremonial register anyway, which consists of speaking in an archaic dialect, Íomanazinení. This is the dialect I've been (until recently) using in the premodern con-chat thread; it has a very conservative word order and very bulky inflections, but few (if any) irregularities, making it easier to pick up than the later everyday dialects. In addition, during the brief Venrafivía slang period, it was fashionable to use the old Zeyetaní dialect for polite speech, a local argolect (ship dialect) for casual speech, and Íomanazinení for ceremonial purposes. The differences between them are chiefly a matter of inflection, although argolects could be more experimental with word order. After Sarasí was standardized, both the Venrafivíai and Zeyetaní rapidly evaporated.

I haven't thought about register in Airumali.

...there's a lot of stuff I haven't thought about in Airumali. ;_;

I really have not worked on register in Thote, but Thote speakers have typically been bilingual, in past times in Tibetan and in present times in Mandarin, and that I imagine that these would have been used/be used for the purposes of an upper register much of the time.

Here's what I have for Nvomi so far:

Nvomi's "recognized" registers can be split into Rough, Casual, Polite, and Formal. The (Written) Standard is generally a mixture of everything but Rough.

Light Verb Bits: These things typically require some sort of "padding".
Formal: Many types of padding, mostly dealing with power relations.
Minkoto. [ work.PAD ] I serve you.
Mingupi. [ work.PAD ] Serve me well.
Dài kyáli tana-rìnhalá. [ GEN\lord SUB\works you-I-IDK\love.PAD ] I have always enjoyed your art, my friend.
Polite: Nine padding words based on time of day, for "short" situations (e.g. talking to a clerk or cashier, passing someone on the street). Otherwise (e.g. chatting at work or school), a few from Formal are used and dropped for the Casual as early as possible.
Casual: Padding varies between people and place; it is always some syllable, followed by a repeat of the bit. Popular with girls for this and Rough is repeating the bit twice. Also common is no padding at all.
Ri! / Riari! / Rikori! / Ripuri! / Ririri! Love it!
Nò(nono)-shi. I didn't say anything.
Rough: No padding used.
Nó-shi. I'm not talking.
Written: Common are shishi, lashi, and shila, combinations of 'be' and 'do'. There's no real rule about using them, but one convention is lashi for light verbs starting with S, SH, or CH, shila for those starting with L, and shishi for everything else. The educated/literary/snobby set prefer these as their spoken Casual padding.

Honorifics:
Formal:
Polite: A couple, mostly based on occupation, used to set the level of conversation and then dropped ASAP.
Casual: Very few, mostly terms of endearment, used like in Polite.
Rough: Varies wildly based on person, area, situation, from a lot to sometimes none used at all.

Pronouns:
Formal: Names and honorifics used with pronouns; inversion of subject/object prefixes occurs with this. Inferiors are sometimes referred to with various demonstratives and bland adjectives.
Myòya mrìu tana-súnhala. [ GEN\Myoya GEN\lady you-I-PRS\see.PAD] I see you, Lady Myoya.
Rùvu ldòa jíshisi mùshishi (tana-)sùngupi. [ GEN\here GEN\servant OBL\go.PAD GEN\come.PAD (you-I-)PST\see.PAD ] I have observed your comings and goings.
Polite: Standard pronouns used as honorifics in short situations (to avoid having to figure out what title or honorific to use). Otherwise, pronouns only used for clarity.
Myòya tári nà-súkosu. [ GEN\Myoya OBL\you I-PRS\see.PAD ] I see you, Ms./Mrs./Lady/I-Don't-Know-Your-Rank Myoya.]
Casual:  Low Nvomi: -e addition to Standard pronouns. High Nvomi: gemination of Standard pronouns. Standard pronouns also common.
Nata-súkosu. / (Low) Tárie/Té ne-súkosu. / (High) Átta/áttari (anna-)súkosu. / (Nerd) Nata-súlashi.  I see you.
Rough: Various dialect words common.
Tòroe ji tòe mu, ne-sù. [ GEN\you go GEN\you come I-PST\see ] I seen ya comin' and goin'.


Regarding Counters, Numbers, Adjectives:
Formal: Adjectives and numbers, in their full forms, must be attached to a counter. Not a general counter, unless the proper counter is actually descended from that (or a similar-sounding) word.
hinonli susu [ one-person son ] one son
haranonli susu [ one-person son ] one son
hinlagi susu [ one-boy son ] one son
haranlagi susu [ one-boy son ] one son
juolyami lyama [ pleasant-season season ] pleasant season
juonsoli lyami [ pleasant-intangible season ] pleasant season
Polite: Numbers must be used in their full form. Adjectives do not have to be attached to a counter, but they must be suffixed with -nga. General counters are okay.
haranonli susu one son
juonga lyami pleasant season
Casual: Counters are only used for clarity; general counters are okay. Short forms allowed for numbers. Adjectives, and numbers, do not need to suffixed to anything.
hara/hi susu one son
juo lyami pleasant season
Rough:  Use counters for clarity (or not!), and in that case only the general ones. Dialect words for numbers are a must.
u susu one son

There's some more stuff with suffixed -e/gemination, which forms diminutives and verb-to-nominals that are usually Casual, the Polite/Formal words that would be used in place of those (formed via compounding), and the lists of words that only belong to a specific register (e.g. most curses are explicitly Rough). Things are further complicated by Nvomi's strict caste system. While Commoners don't have to worry about too much, the Nobility have it rough, especially in the Honorifics department. But that is mostly for Formal situations, which there is always time to prepare for.
There'll be stuff for a few areas where Polite and Formal are merging, but I haven't worked any of that out much. Right now it's mostly diphthongs = Polite, new monophs = Casual; emphasis on pitch accent = Polite, emphasis on volume/vowel length= Casual.

An idea for how to make this thread thrive: build a translation challenge out of it. I'll draw up a list of plausible core words and phrases that might be common register markers.

So much spam. Wow.