Formatting numbers using current locale

Summary

GitLab displays numbers in a number of places (😄). From data tables, to stats and data, some users such as admins us them a lot.

Should we agree on a "global" way to format numbers according to user defined locale? Which locale should it be?

The problem

Numbers are formatted using differently depending on locale:

Value	English	Spanish
1000	1,000	1.000
2.95	2.95	2,95

Possible solutions

🅰 Use GitLab's language as locale

GitLab's user settings allow users to select a language.

Example: The selected language sets the document lang, so we could do:

{{ (1000).toLocaleString(document.documentElement.lang) }} <!-- A `utils` could do this for us :) -->
{{ (5.1).toLocaleString(document.documentElement.lang) }}

🅱 We use the browser's language

We assume users' have configured their preferred locale in their browser, so we use the browser default formatting.

Example:

{{ (1000).toLocaleString() }} <!-- locale defaults to OS/browser settings --->
{{ (5.1).toLocaleString() }}

🤷 We leave it as-is

We keep rendering numbers as always.

Example:

{{ 1000 }} <!-- This will simply JS rendering: "1000" --->
{{ 5.1 }}

Expect RFC outcome

Gather info to document the recommended way to render numbers in our UI in the GitLab frontend guidelines.

Further development work would help enforce the guidelines with libs, linters, etc... that help us get it right easily.