Why does the "admin/modules" page show up in the wrong language?
I did a similar verification in one of our sites, which has 3 languages (EN, NL and FR). I disabled the "module_filter" module, so that I could see what the result would be if I look at what's shown on admin/modules. I also use "admin menu", but for my verifications I've not experimented with anything related to that menu (I just wanted to check what the CONTENT of  admin/modules actually is, depending on the language I used. And I experimented with EN and NL (=Dutch), assuming that FR would give similar results like NL. And I've been using the (same) Seven theme for admin

To actually do my experiments, I've enabled the "language switcher block" so that it is shown within my theme used for Administration. I have this block shown in the "Content" region above the block "Main Content Page" (so that I can switch between EN and NL for my experiment). BUT: nowhere in the question here it says anything about using that "language switcher block". So that's already something for you to check: you are aware I assume about how to use that block for switching languages, right?

Anyway, when I then use that language switcher block to switch from EN to NL, PARTs of what is shown on that page is in NL (Dutch), though other parts remain in EN. E.g.:


the descriptions for my modules (from the .info files) are shown in Dutch
but the strings like "Requires" and "Required by" are NOT translated in Dutch. Though the values like "(enabled)" or "(disabled)" or also translated to NL, like to "(ingeschakeld)" or "(uitgeschakeld)".
the (hardcoded) tekst near the top, like "Download additional ..." is NOT translated.
typical messages (in green, yellow or red) near the top are translated to NL also.
Module names, like "Color" are not translated (to something like "Kleur").
links like "configure" are also translated to "instellen".
module groups like "Core" are translated to "Kern".


With all this, I have the impression that by using the "language switcher block" and setting it to the NL-language, this admin/modules page gives me an "as good as it gets" translated version of that entire page. But if you don't use that  "language switcher block" (to switch to NL), then everything remains in EN (of course). And for things that are not translated, I guess that's because it's either not using the typical t() function, or the required translated string is still missing (for NL in my case).

Not sure to what extend it will actually answer your question, but at least it should help to understand what is happening, and how the "language switcher block" seems to be part of the puzzle.

But, don't worry ... D8 has a lot of improvements for multi-language stuff (not sure yet about this specific "issue" however ...).