Q:

How to choose an easy language to learn based on the languages that I already speak

We know a lot of great things that can motivate us to start learning a new language, boosting up our career, having a great travel experience, etc. But what would be some techniques that we can use to choose a new language to learn based only on our native language and the languages that we have already learned?
I speak Tamazight, Arabic, French and English (I am not fluent in all of them), and I just started learning German. As I mentioned above, I know a lot of things that motivate me to learn languages, but my question is specifically about some techniques that we can use to choose languages to learn that might be easy based on what we have already done.

A:

There are several criteria you can use:

Mutual intelligibility. You can pick a language that is mutually intelligible with one of these you already know. For example, if you know English, a good candidate would be Scots. If you know German, you may consider learning Dutch or Yiddish.
Pidgins and creoles. These are languages that developed in the situations of lack of a common language between speakers of different languages. They tend to have simple grammars, which makes them easy to pick up, especially if they are based on language(s) that you already know. For example, you might be interested in learning of one of the Arabic-based, French-based or English-based creoles.
Genetic relationship. Languages that share a common ancestor tend to have common words, similar grammatical structures, etc. For example, knowledge of French is certainly an advantage while learning Spanish, Italian or Romanian, even though these languages certainly aren't mutually intelligible with French.
Cultural influence. For example, even though Chinese and Japanese aren't mutually intelligible or genetically related, Japanese has a huge number of Chinese loanwords, and its writing system uses Chinese characters. If you know one of the languages, learning the other is definitely much easier than if you don't. Another example is Malay, which has been culturally influenced by English, due to the colonial period.
Writing system. It is much easier to learn the language if it uses a writing system you already know well. You can use this advantage if you know Arabic and learn Persian, or if you know Japanese and learn Chinese.
Availability of learning materials. If you want to learn an uncommon language, you may face the problem of lack of good quality learning materials in languages you know, so you need to take that into account. For example, if you know Tamazight, it should be quite easy for you to pick up Ghomara, but I'm not sure if'd be easy to get high-quality learning materials.
Constructed languages. They are generally simpler than natural languages, and their grammar and vocabulary is usually easier for speakers of some langauges than others. For example, Esperanto should be easy for speakers of most European languages and Slovio is constructed to be easy for speakers of Slavic languages. 
Inherent simplicity. Creoles, pidgins and other languages that have been used as lingua franca (including Malay and Chinese) tend to have less complicated grammar than others. This is also a factor that you should take into account if you want to choose a simple language.

