What type of sampling method is it if I choose people who are between 18-22 and speak a specific language? I would like to know how it is called as a sampling technique when I need people that are between certain age range and able to speak German. The research is an experiment and people are asked on the street if they want to participate or not. Then, I ask if they are between 18-22 and if they can speak German. I ask people who I think they are between 18-22 in order to be more efficient and save time. Is it called homogeneous sampling (purposive) because research is all about people who are between 18-22? Or convenience sampling?
 A: This doesn't appear to be an example of quota sampling, at least not from the limited information you give. You may be confusing your population of interest with the sampling method, in fact. 
Quota sampling is similar to stratified sampling, in that you are essentially reproducing the composition of a wider population, except (unlike in stratified sampling) you don't have known probabilities for each individual in advance. If you were studying some population of interest and German speakers aged 18-22 within that population were a group of interest that you wanted to study, you would set a quota for them and continue interviewing German speakers aged 18-22 until you have filled the quota—that would be quota sampling. 
However, it sounds like German speakers aged 18-22 are your entire population of interest; i.e., you are only interested in them. Now, whether your sampling is purposive or not depends on how you select your subjects / participants. Purposive sampling usually involves a subjective decision by the researcher to include an individual in the sample, and it is usually done when both the population of interest and the sample are small. For example, if you are studying German speakers aged 18-22 in, e.g., Britain or in one town or in one university, maybe you will collect your sample through a personal network or by snowball technique. In these cases, you would know your population of interest is relatively small, so probability sampling would not be adequate. However, I'm wondering if you're interested in all German speakers aged 18-22 in the world (including those in countries where German is an official language). That's a lot of individuals, so keep in mind that purposive sampling may not be an adequate sampling method to say something about them, especially if your research is quantitative and you want to obtain some point estimates. 
