Translations are supported, but they happen before substitutions. So you won't be able to dynamically generate a language key like that. You can also just use IF substitutions to condition different text. Example as follows.

[cb:if var3="1"]
Plan 1
[cb:elseif var3="2"]Plan 2[/cb:elseif]
[cb:elseif var3="3"]Plan 3[/cb:elseif]
[cb:elseif var3="4"]Plan 4[/cb:elseif]
[/cb:if]

See substitution below for further substitution usage information.

www.joomlapolis.com/documentation/279-community-builder/tutorials/18353-using-substitutions-throughout-cb

If we substitute 5 different text strings for 30 plans id, will it still be ok to work with or would it be too slow to handle ?

You can substitute as much as you want. No, it won't be slow to handle.

Do you have a timetable for CBSubs URL fix ?

I don't know. I'm still working on recurring payments for Stripe.

BTW : would cbif works with CBSubs gateways ? for instance could I condition the owner to the status connected of a user or better to a plan id ?

I don't understand what you mean. Again, you can substitute all the variables the trigger provides. Beyond that you've regular profile substitutions. If you need to execute functions you can do so using Format Functions.

Ok, rewrote the CBSubs URL integration to use Guzzle. There's also a new parameter to specify the Get body instead of having to cram all the variables into the URL (both will work, but the new param is easier to read and setup). You'll also be able to add header variables as well. These changes are still pending review and once merged a new nightly will be released.