Hi, welcome to SoSci Survey! You can do nearly everything with SoSci Survey ... but it will require a bit PHP code sometimes. Such in your case :)
1) Is it possible to randomly show a certain number of questions (8 out of 15)?
This manual is a good start für your question: Rotate Questions
The only thing to do differently than in the manual: Draw only 8 instead of 15 codes per interview. You will set that in the random generator.
If the languages are just items of a question, it may be even easier. Draw the 8 out of 15 codes by the random generator (like before), and use this PHP code:
$items = array_keys(valueList('RG01'));
question('FQ01', $items);
RG01 is the random generator in this example, and FQ01 the followup question with 15 items of which you want to display 8.
2) Is it possible to make sure that for the next set of questions, the same 7 languages will be addressed, and that a new random selection of 8 won't be done automatically?
The random generator will store the 8 codes. You can use that data for any followup questions. However, if you actually have 15 distinct questions (not just items), a bit more PHP code will be necessary to make (for example) CD02 and CD05 from AA02 and AA05. But that shall be the issue in another question.