Andrew,
I translated Del as Удалить yet this word appear in dropdown menu (Edit->Clear-> Clear (Del) ... ) in main window, where (I think) it shouldn't be translated. Can this be fixed by marking it  as untraslatable?

Yes, for example look in ru.po to see how the following are handled + look at the code since a small code change is needed as explained below.  It is easy to change and we did this at the request of IgorUbuntu as needed in the past.  But of course, you can supply the patch yourself.

    sketcherwindow#Del
    tracerwindow#Del
    crikeywindow#Del
This is documented in the Manual in the Translation chapter but it is a little difficult to comprehend -

NOTE: some words and abbreviations can lead to ambiguous language trans-
lations. Therefore, the usage of C_ and D_ in the program code was added to
represent Contextual and Definitional exceptions to the usual _ and N_ . You will
see the following:

C_(“msgstr”) is translated to D_(“qual#msgstr”) by xlat.sh, and invokes get-
text with msgid = “qual#msgstr”

When no po translation is supplied, the qual# is removed, and only the default
msgstr text is displayed. If a po translation is defined for the current locale, then

the translated msgid = “qual#msgstr” is used to access the translated msgstr.

The default MSGQUAL is the basename of the C source file. For the file src_file.C,

the default MSGQUAL is:

# define MSGQUAL “src_file”

It is used to define the qualifier needed to transform:

C_(“str”) to D_(“src_file#str”)

The resulting xlat.sh’d source is scanned by xgettext to create the initial cin.po. In
other words:

_(s) does normal international text translation as always -

The msgid line is: msgid “s”

C_(s) makes it appear as if you used D_(“src_file#” s) -

The msgid line is: msgid “src_file#s” If it does not translate, the default
msgtext is “s”, not “src_file#s”

D_(qual#s) tries to look up _(qual#s).