Catalogue searches:
MultiSearch & databases search terms try:
"Second language acquisition"
"Second language learning"
For minority languages try:
"linguistic minorities"
"language minorities"
"minority languages"
"heritage language"
For language revival try:
"language revival"
"language revitalization"
"language revitalisation"
"language maintenance"
For language loss try:
"language attrition"
"language loss"