Since SEBLOD is (almost) well rounded regarding multi-language usage, I wonder have anybody found a solution how to use placeholder attribute in multi-language scenario. JText does not parse placeholder text, so there should be another solution (other than creating alias fields for every language or even worse using JS onclick).
Similar code can be placed in other text fields, inserting/substituting just the field name and the override string.
P.S. Of course, since there is no need to duplicate override strings, it is perfectly ok to use COM_CCK_strings. However, as placeholders our labels can contain additional explanations, examples and so on.
Even though I haven't tested it, I suppose your solution is very nice for automation, but you skipped to say where do you put various language translations for placeholder. I imagine it should be some label (a label one language each) in the same group, that should be language constrained.
the label is the placeholder. For example you put an "test" in it and you have a multilanguage site, the output of the placeholder is "COM_CCK_TEST".
Then you have to translate it in your desired language files. In the english file en-GB.com_cck_default.ini with a COM_CCK_TEST="English test" and in the german file de-DE.com_cck_default.ini with a COM_CCK_TEST="Deutscher Test".
When you work only with one language, the output of the placeholder is "test" and you don‘t need to translate it.