Hello, here are some supported languages:
“lang_ar”: Arabic
"lang_bg": Bulgarian
"lang_ca": Catalan
"lang_cs": Czech
"lang_da": Danish
"lang_de": German
"lang_el": Greek
"lang_en": English
"lang_es": Spanish
"lang_et": Estonian
"lang_fi": Finnish
"lang_fr": French
"lang_hr": Croatian
"lang_hu": Hungarian
"lang_id": Indonesian
"lang_is": Icelandic
"lang_it": Italian
"lang_iw": Hebrew
"lang_ja": Japanese
"lang_ko": Korean
"lang_lt": Lithuanian
"lang_lv": Latvian
"lang_nl": Dutch
"lang_no": Norwegian
"lang_pl": Polish
"lang_pt": Portuguese
"lang_ro": Romanian
"lang_ru": Russian
"lang_sk": Slovak
"lang_sl": Slovenian
"lang_sr": Serbian
"lang_sv": Swedish
"lang_tr": Turkish
"lang_zh-CN": Chinese (Simplified)
“lang_zh-TW”: Chinese (Traditional)

And here is the list of supported countries:
https://developers.google.com/custom-search/docs/json_api_reference?hl=en