Even if ubiquity is doing an awesome job at installing on demand languages on ubuntu, having more default languages as we used to on the destkop image enables us to have the live session localized. Also, offline installation will bring that support without adding supplementary download time.
Some loco team used to have respin of the default localized ubuntu image. Yours truly used to have handle this quite regularly for the french loco team. I know that the italian and finish locos used to do it as well (not sure about the current status for them). However, UEFI support made it complicated and so those efforts have been dropped some cycles ago as far as I know.
I would thus propose that we enhance and complete our default language strategy for xenial. The LTS is an excellent time to make progress on this.
The proposal is to:
1. Install full language support for those shipped on xenial image. It means that opening "language selector" won't request any additional package to install. If you are proceeding an online installation, additional packages won't be downloaded to complete your language installation. If you have done an offline one, you won't have the infamous after first boot "Language support is not complete" dialog. Note that for now, we have no complete language support on the live! For instance, in English, we have the following missing packages that language-support will require to install (or that ubiquity will download it for you if you are connected to the Internet):
hyphen-en-us, mythes-en-us, mythes-en-au, hunspell-en-ca, myspell-en-au, myspell-en-gb, myspell-en-za, libreoffice-
2. Based on popcon, number of native speaker and total number of speaker per language, it seems that the following language selection makes sense for our user base (more info on the language selection on https://bugs.launchpad.net/ubuntu/+bug/1520278):
en, es, zh (simplified), pt, de, fr, it, ru
3. We remove the difference between archs for now on the ubuntu desktop live seed (no language installed only on i386/powerpc like German and Portuguese, where they aren't installed on amd64), no more partial libreoffice help support only installed on i386 and amd64, for consistency.
Additional estimated size on the live to achieve this is 95Mb adding the full support for those 8 languages. This is of course a non negligeable image size increase. However, getting full language support for at least 8 languages (and no additional download on install for those) seems like a huge increase in quality to me. I'm sure as well that with the python2 drop support and other cleanup efforts, we would be able to regain a great part of that additional iso size (remember that our image was still at 1Gb not so long ago where we are at 1.2Gb now).
Finally, keep in mind that those locales are only part of the live image. The chosen language will be present on the installed system, but no other ones, even if shipped on image. So no extra space will be required on the installed target.
More rationale is available on https://bugs.launchpad.net/ubuntu/+bug/1520278. If there is no opposition to this, I propose to make the changes in the coming days and then, and we can always reassess of course if needed.
 apart from one font for simplified chinese which is weighting 8Mb.