I place a common long inserter on the ground. Then I take a vanilla upgrade planner and tell it to replace the long inserter to Schall's Fast inserter II. For some reason the long inserter is not marked for replacement, despite it hasthe same length as Fast II. However, if I place bulk inserters, the same (long -> fast II) planner replaces bulk inserters with the Fast inserters II.
Is it some of my mods fault?

I did not expect the newly-added inserter series to have any relations to vanilla inserters. They fit vanilla fast inserter but do not fit the others too well together (in terms of tech tree, recipe relations, etc).
Therefore, I have not set any lines of code about fast_replaceable_group nor next_upgrade properties in my mod. (Which defaults to "inserter" group without assigning upgrades.)

Vanilla game have the long-handed inserter in another group "long-handed-inserter" which is not shared by any other entities, so it is not shown in upgrade planner. Actually I doubt if it is a good way. (Forcing different group for each range does not sound useful, especially when no upgrades are set.)
Rather I prefer the long-handed inserter to be put in line with all other inserters, letting players to freely replace to whatever they want. But I have chosen not to modify any vanilla inserters, yet... Should I do that?

As a common player I comprehend Fast inserter II as an upgrade to a Long inserter. Because from a common player perspective they differ only with speed. The code behind that is far out of my understanding (and, frankly speaking, out of my interest).

While experimenting I did notice, that Fast inserter II can replace a Fast inserter, but, to tell the truth, I did not expect that. This possibility is cool, but not obvious.
Silent upgrading Bulk inserters with Fast inserter II instead of directly specified Long inserters feels like a bug. It is absolutely not obvious.

Replacing any inserter with any is quite acceptable. Personally I use Cursed-PI, so interchanging a longer inserter with a shorter one is not something surprsing. I suspect the popular Bob's inserters mod also allows that. If needed, you might also check compatibility with Modular turrets's asymmetric Armored inserter.
A group for each range... well, this probably is unnecessary, if I understand your doubts correctly.

I have implemented an option to change Long-handed inserter back to the default inserter group (like all others). Enabling it also allows upgrading, where I put in the next_upgrade code to Fast inserter II.

Then there is extra text needing translation. Thanks in advance.

[mod-setting-name]
Schall-long-handed-inserter-group-same=Same group __ENTITY__long-handed-inserter__
[mod-setting-description]
Schall-long-handed-inserter-group-same=Put __ENTITY__long-handed-inserter__ into the same replaceable group as all other inserters.\nOn: Upgradable to __ENTITY__Schall-fast-inserter-R2__, or replaceable to other inserters.\nOff: (Default) In separate group.

These will be included in the coming release on Thursday (my usual update schedule), if everything goes smoothly.

PS: Nice, I am using Cursed-PI too. So you will understand why I don't like one group for each range. :-)
There could be different "policy" in different inserter mods, it would be messy to detect which mod and use different policies for each combination... I think it would be better if I stick to policy of one big group for all inserters.

The translations are updated in my mod.

Just released 0.17.4, which provides the requested features.

PS: There are some minor mistakes in ru locale. I have made some corrections to it.

Oops. That's why I don't like translating without being able to test.

Oh, actually I expected you to paste the translated text here, so I can put them to the concerned mod directly.
Anyway, nvm.