This is called "velar nasal" consonant ng’ /ŋ/, it is just a different sound of "n" (and the "ng" is a "prenasalized sound of "g", /ᵑɡ/).
ng'ombe [ŋɔmbɛ] 'cow'
(here the "b" is prenasalized with "m", so ng'o has one sound of consonant, and mbe has two sounds "m" + "be")
So, if I understand you here, the usual NG is a velar nasal followed by a G sound, as in "finger," while this NG' is simply a velar nasal alone, as in "singer."
The example for English would be "sing", and yes if you use "finger" as example, this would be pronounced fi + nger (as you said with a velar nasal alone). Sorry, I still do not know how this word is pronounced by English speakers because I am a native Spanish speaker. Other example I can mention is the Indonesian sound of ng and it is just the same as Swahili ng' (the Google TTS for Indonesian will bring a better demonstration with words as "bunga" = flower, "pasangan" = partner, mate, duo, "orang" = person, "orang utan" = orangutan, and the counterpart will be the combined sound of "ng" + "g" in "tinggi" = tall, high, or "tangga" = stairs).
Edit: The apostrophe is really positioned after the two consonants ng'. It is my fault, and I will fix that. You have pointed very well with the Malay term below. Absolutely yes! =)
Ah, yes, and you really do not have this sound in Spanish. I'm afraid I do not know what a Google TTS is, though I am familiar with the Malay term orang, and I assume the Indonesian is the same. In Swahili, then, the NG alone would be pronounced as two sounds, like the Indonesian NGG, it sounds, or like the English word "finger," which would, I guess be *fingger written out for an Indonesian to pronounce. I do note also, though, that you put the apostrophe in a different place than this program does. You are writing N'G quite consistently, while the program always writes NG'.
Na asante wewe (I have no idea whether that is grammatically proper). This does clear it up for me, and now I see what the TTS is, so I have a great deal to thank you for. I also doubt any relation between the Swahili G and any sort of click languages, as there really does not seem to have been any geographical overlap, even in the early development of Swahili. Could it actually be an influence from Arabic, which does have a variety of plosive sounds? Of course, I have no idea what the Omani dialect might have sounded like at any point. The GH in the Italian words Ungheria and funghi actually sounds to me like the English hard G that I had thought all Swahili Gs were. And if you are interested in Austronesian languages, then Malagasy should be fascinating. My daughter had a Malagasy-speaking friend for a brief time, and when her parents spoke, it sounded for all the world to me like standard Malay. I did not know of Arabic or Bantu influences, but that really would make it the Indian Ocean language par excellence.
Now I have edited the apostrophe. Asante sana rafiki!
TTS is a program, I think it is called a motor for the synthesized voice or Text-To-Speech in Google Translate.
(The Swahili motor does not show a proper sound in the examples of words with ng' just the English names "n" and "g" separately. But still it is good to hear the stress and basic pronunciation in almost all the Swahili words. Of course, the pronunciation cannot be compared to any Swahili dialect. Also, I have noted some Swahili speakers as pop singers are using different pronunciations, sometimes they pronounce a t sound as English speakers. I think this sound is like in Spanish or Italian "toro" or "latina", but the g sound alone is really different from many languages. It is neither like Spanish "guerra" or "galaxia" nor "gema" or "agilizar", but the Swahili sound is plosive, perhaps between k in English "cat" and g in Spanish "gato", but never the same as in Spanish. Other Swahili sound is gh and this can be the same as Italian "Ungheria" or "funghi". Also I guess if the Swahili g can be related to a click sound as in Xhosa but I do not think so, and I do not know about what relationship would be between these African languages. I am really interested more in Malagasy because it is from Austronesian roots and also related to Bantu and Arabic languages.)
Yeah, basically "finger" with my Australian accent would be written finga and "singer" would be sing'a.
Off topic: Is that not the standard pronunciation of English words ending in -er? I thought the rhotic R was just a North American thing (and maybe sth occurring in a few cities in Britain)
I think a lot of West Country accents in England at least used to have a rhotic final R. On the other hand, parts of the Southern and Northeastern United States do not have a final rhotic R. That said, I'm not sure what you mean by "standard" here. I don't think the UK has a language authority like France does. In any case, I have no idea why the English of the 66 million or so inhabitants of the UK would be considered any more standard than that of the 363 million or so inhabitants of the United States and Canada. That is not to mention the English speakers of Australia, New Zealand, the Caribbean, Africa, India, etc.
Diego, as far as I know, many Swahili celebrities (and politicians) mix Swahili and English (words and pronunciation) because they think it's kind of posh, as if it was a "more educated" way of talking (been told by Swahili native speakers).
Oh, y felicidades por tu inglés.