Based on my learnings from localizing applications in SvelteKit with svelte-i18n, sveltekit-i18n and typesafe-i18n, I've come up with advantages and limitations of each of these libraries.

svelte-i18n

Features

• It is a simple and minimalistic library that uses formatjs for localizing messages and supports the ICU message syntax.

• It offers to load translations in a synchronous as well as asynchronous manner as per requirement.

• Provides a decent number of options for formatting and pluralization.

Ease of Use

• Since this library uses native Svelte stores for implementing localization, it feels like part of the ecosystem and hence very easy to get started with!

• If you have a simple requirement of changing translations based on a given locale with limited formatting, this should be your go-to library.

Size

• As of v3.7.0, the size of the svelte-i18n package is 48.3 kB in the minified form and 14.2 kB in minified + gzipped form. (source)

Community Support

• It has 29,978 downloads/week as of the time of writing this article.

• The only downside is, this library is not actively managed. Several issues are on the rise and resolution is almost zero.

Limitations

• There are very less options available for formatting and pluralization.

• As mentioned before, the library is not actively managed.

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sveltekit-i18n

Features

• It is built for SvelteKit and has good SSR support.

• We can load translations from API, database and local file system.

• The translations are loaded for visited pages only. (and only once!)

• Component-scoped translations: you can create multiple instances with custom definitions.

• It provides good TS support and has no external dependencies.

• Supports ICU syntax + provides a native parsing engine as well.

Ease of Use

• It is very easy to use. I found it even easier while working with the ICU parser (instead of the default parser).

Size

• As of v2.4.2, the size of the sveltekit-i18n package is 14 kB in the minified form and 4.6 kB in minified + gzipped form. (source)

Community Support

• It has 3,052 downloads/week as of the time of writing this article.

• Even though the number of downloads is less than the svelte-i18n but good news is that this library is actively maintained.

Limitations

• While working with pluralization, understanding the syntax of the default parser could be a bit challenging at the start.

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typesafe-i18n

Features

• It is a lightweight, easy-to-use syntax and has no external dependencies.

• Because of its strong typings, it prevents you from making mistakes (also in plain JavaScript projects).

• Unlike the other two libraries, typesafe-i18n can be used with any framework and supports both TypeScript and JavaScript.

• Creates boilerplate code for you which ensures the well-structured flow of i18n in your codebase.

• Supports plural rules and allows formatting of values e.g. locale-dependent date or number formats.

• It also supports switch-case statements e.g. for gender-specific output.

• Provides an option for asynchronous loading of locales.

• It supports SSR (Server-Side Rendering) and can be used for frontend, backend and API projects.

• Import and Export translations from/to files or services.

Ease of Use

• It has some initial learning curve but as soon as we become familiar with the typesafe-i18n ecosystem, this library is not only easy to use but super convenient as well!

• Personally, this is my go-to library for i18n.

Size

• As of v5.26.0, the size of the typesafe-i18n package is 2.8 kB in the minified form and 1.3 kB in minified + gzipped form. (source)

Community Support

• It has 13,588 downloads/week as of the time of writing this article.

• This library is actively maintained.

Limitations

• It has a huge eco-system (generators, detectors, importers, exporters etc.) and hence learning curve is a bit steep.