LGPL specifically does as far as I understand have some issues when used in rust. In particular the border for the copyleft is dynamic linking. That doesn’t work well with rust. I would instead consider MPL where the copyleft border is on a source file level.
I really don’t see what niche it is trying to fill that isn’t already occupied.
Rust is as successful as it is because it found a previously unoccupied niche: safe systems programming without garbage collector and with high level abstractions that (mostly) optimise away.
I don’t think “better C” is a big enough niche to be of interest to enough people for it to gain a critical mass. I certainly have very little interest in it myself.
There are existing approaches: GNU gettext and Mozilla fluent comes to mind. I would try to use one of those. I understand that Mozilla Fluent has good support for the Web (unsurprisingly).
Your idea will work with minor changes (if comments are supported in your file format). At work our tooling create entries like 123=“English text” // UNTRANSLATED. Obviously not quite the same format, but it should be adaptable to any format that supports comments.
Be sure to treat state and configuration separately. It doesn’t matter on Windows as far as I know (they go in the same location), but on Linux those are supposed to go in different places.
Many programs get this wrong, and it is quite annoying as I track my config files in git. I don’t want a diff just because the list of recently opened files changed! Or even worse: the program stores the last window size and position in the config file… (looking at you KDE!)
Here are some libraries I found to help with this in a cross platform way:
I haven’t tried either, haven’t written such a program yet.
As for how to store data, there are indeed many options, depending on your needs:
Plain text based formats (toml, yaml, JSON, ini, …) can be good for configs and basic state. As a bonus it let’s the user easily manage the file in version control if they are so inclined.
Databases (SQLite mostly)
Custom formats (binary files in a directory structure is often used for browser caches for example) .
Without knowing more it is hard to give specific advise.