Depending on your location.
For all non English speakers, English should be the first secondary language that is learnt.
The choice can be simplified as follows:
Spanish is only really usefull if you live in America. Spanish is not widely spoken outside of Spain (with the exception of the Americas) and can be considered fairly useless in Asia where no nations use Spanish as a primary language.
The most efficient way to learn a new language is through consistent practice, immersion in the language, using language learning apps or courses, practicing with native speakers, and setting specific language learning goals.
American Sign Language (ASL) is the most widely used sign language in the United States and Canada. It is recommended to learn ASL as it is a valuable skill that can help you communicate with Deaf individuals in these regions.
The most common foreign language people learn in the UK is French. It is widely taught in schools and is often seen as a valuable skill for business, travel, and cultural purposes.
The hardest European language to learn is subjective and can vary depending on the individual's native language and language learning experience. However, languages like Hungarian, Finnish, and Icelandic are often considered challenging due to their complex grammar rules, unique vocabulary, and unfamiliar phonetic systems.
The language most people want to learn would ether be English or Spanish. Most foreign countries are teaching their children English along with their native language because America is the current world superpower. Most American people want to learn Spanish because it is easily translated to English and is possibly somewhat romantic to some people. Also Spanish and English are simple to most people.
English
Most of Indonesian learn Indonesian language as second language aside of their local regional language.
Most Brazilians, like most other people, don't go anywhere to learn their native language. They learn it at home as infants and young children.
they are all desirable but the SS is the most desirable
the most widely spoken language is english so that is good to learn if you don't know it already. spanish is also a good language to learn because it is common and the most people on earth speak chinese so that is also good.
The most efficient way to learn a new language is through consistent practice, immersion in the language, using language learning apps or courses, practicing with native speakers, and setting specific language learning goals.
The best language to learn singing in Europe is English because it is the world's most spoken language.
hindi
You can learn a slavic language online by using the slovio site or the multilinguals website. Most of the available options are not free and they usually connect you with a personal tutor who will help you to learn the language.
Probably Mandarin Chinese.
Most experts believe immersion is the fastest way to learn any language.
English is the official International language for all business communication and all airline communications therefore it is the most useful language to learn as a second language.