Three different things called voice on one Galaxy
Before anything else, work out which piece of software you have actually been using, because on a Samsung phone there are three of them and they are reached the same way.
A Galaxy typically offers the keyboard's own voice input, the platform's voice typing, and an assistant. All three start when you speak at the phone. They are separate systems with separate settings, separate behaviour when the connection drops and separate answers to the question of where the processing happens.
Advice about Samsung dictation contradicts itself constantly for that reason. Two people can both be describing "voice typing on a Galaxy" and be talking about entirely different software, and both of them can be right.
Open a text field, press the microphone, and look at which interface appears. Whatever you then conclude about privacy or offline behaviour applies to that one and not to the other two, including anything you conclude from this page.
Where Samsung already gets this right
Plenty of pages on this subject are unfair about software that came with the phone.
The Samsung Keyboard on Galaxy phones supports offline voice input once you have downloaded a language pack, it is free, it arrives with the handset, and it does not require a Samsung account. On a recent Galaxy that is a perfectly reasonable answer, and if it works for you there is no argument here.
By default the keyboard points at the platform's voice input rather than at the offline pack. Offline behaviour is something you go and enable for yourself; it does not arrive switched on. It does exist, though, and it does work.
The ledger runs further than that. Samsung's keyboard is maintained alongside the phone it shipped on, it supports far more languages for typing than eighteen, it has learned from years of your own typing, and it costs nothing, where LocalType is paid after a trial. A Galaxy owner weighing those against one narrower guarantee is doing entirely sensible arithmetic, and plenty of them will decide to stay.
A mode, or the only mode
With packs, whether your speech is processed locally depends on which language you are speaking, whether the right pack finished installing, which handset you have, and what a future update decides. Each of those is fine on its own. Taken together they mean the honest answer to "is my audio being transmitted right now" is "probably not, assuming everything is still as I left it".
LocalType does away with the question. There is no remote path for speech in the app at all, so there is no state to be in, nothing to verify before a sensitive sentence, and nothing that can quietly change while you are not looking.
Whether that matters depends entirely on why you wanted offline dictation in the first place. For patchy signal on a train, a pack is fine and it will fall back without complaining. For not wanting your voice transmitted, a setting is a weaker guarantee than an absence.
Packs versus one multilingual model
A language pack is one more thing to keep track of. You install them per language, you keep track of which are present, and you find out one is missing at an inconvenient moment. Each also occupies storage of its own.
LocalType covers eighteen languages with a single multilingual model, downloaded once, and works out which of them you are speaking from the recording rather than from a menu. Start a message in one language and the next in another, and it follows without anything being selected.
Set a specific language instead and the keyboard layout, the dictionary and the dictation language all move together, which is the better arrangement if you will be in one language all day.
Accuracy is not identical across those eighteen. Training material is unevenly distributed and widely spoken languages do best, which is true of any speech model regardless of who built it.
What changes if you switch
The recognition stops being configurable and becomes fixed. There is one path, it runs on the handset, and it is the same path for every sentence in every language.
That single decision produces the rest of the list. Nothing needs signing into, so no account exists and no identity is attached to your typing. The product carries no advertising, and no behavioural tracking runs inside it. There is no archive of what you dictated, since the audio becomes text and then stops existing. The model sits in storage only this app can reach, Android's cloud backup is deliberately denied the app's data, and over a password box no microphone key appears. The app asks for internet access for exactly one purpose, collecting the model you chose, and has no second use for it.
You also get a decision the manufacturer normally makes on your behalf: three model sizes, at 60, 190 and 539 MB, trading speed against accuracy. The smallest suits an older or busier handset, the largest suits noisy places, and only one lives on the phone at a time. On the 4 GB test phone the middle setting turned 6.9 seconds of speech into text in 4.1 seconds, while the largest takes roughly twice as long as you spent speaking.
Away from the microphone it is an ordinary keyboard, with corrections as you go, the next word suggested, a dictionary matched to your language, the layouts people expect in the countries that use them, a full emoji set, and space for your own names and jargon. Those additions stay on the device and are given to the speech model as context, so they start arriving spelled correctly when you say them out loud.
Your next Galaxy will arrive without any of it
Samsung owners move between Galaxy phones more often than most people change brands, and the transfer usually restores everything they had.
It will not restore this. LocalType keeps its data out of cloud backup by design, so a new handset arrives without your model and without the words you added, and you set it up again by hand.
Anything that reappears on a new phone by itself was stored somewhere other than the old phone. That is the trade, stated as plainly as it can be stated, and it runs in one direction only.
The practical answer is to plan for it rather than be caught by it. Download the model over Wi-Fi while the new phone is still on the sofa next to the old one, and put back the handful of names and technical terms you type most often before you need them. Doing it deliberately on day one is a great deal less annoying than discovering it mid-message on day three.
Nothing to uninstall if you hate it
Android allows several keyboards to be enabled at once, and moving between them is one tap from the keyboard selector.
So the stock keyboard stays exactly where it is. Enable a second one, use it for a week or two, and fall back mid-sentence whenever it irritates you. There is no uninstall step in the experiment and nothing to undo if you decide against it.
If you want offline dictation on a Galaxy phone and are happy managing a language pack, the keyboard already on the phone does that.
If you would rather it were not a setting at all, and you write in more than one language, that is the gap LocalType fills, and a fortnight of running both is how you find out whether the gap was one you actually felt.