On Transifex the languages have an underscore syntax: xx_XX
In Joomla we use a hyphen syntax: xx-XX
It is a real drag to have to rename all those xx_XX folders and files to xx-XX syntax before you can use them for Joomla.
Luckily you don't have to!
You can use language mapping in your tx client config to do this conversion.
Just add a lang_map to the [main] portion of your config file. So like:
[main]
host = https://www.transifex.net
lang_map = af_ZA:af-ZA,am_ET:am-ET,ar_AE:ar-AE,ar_BH:ar-BH,ar_DZ:ar-DZ,ar_EG:ar-EG,ar_IQ:ar-IQ,ar_JO:ar-JO,ar_KW:ar-KW,ar_LB:ar-LB,ar_LY:ar-LY,ar_MA:ar-MA,ar_OM:ar-OM,ar_QA:ar-QA,ar_SA:ar-SA,ar_SY:ar-SY,ar_TN:ar-TN,ar_YE:ar-YE,arn_CL:arn-CL,as_IN:as-IN,az_AZ:az-AZ,ba_RU:ba-RU,be_BY:be-BY,bg_BG:bg-BG,bn_BD:bn-BD,bn_IN:bn-IN,bo_CN:bo-CN,br_FR:br-FR,bs_BA:bs-BA,ca_ES:ca-ES,co_FR:co-FR,cs_CZ:cs-CZ,cy_GB:cy-GB,da_DK:da-DK,de_AT:de-AT,de_CH:de-CH,de_DE:de-DE,de_LI:de-LI,de_LU:de-LU,dsb_DE:dsb-DE,dv_MV:dv-MV,el_GR:el-GR,en_AU:en-AU,en_BZ:en-BZ,en_CA:en-CA,en_GB:en-GB,en_IE:en-IE,en_IN:en-IN,en_JM:en-JM,en_MY:en-MY,en_NZ:en-NZ,en_PH:en-PH,en_SG:en-SG,en_TT:en-TT,en_US:en-US,en_ZA:en-ZA,en_ZW:en-ZW,es_AR:es-AR,es_BO:es-BO,es_CL:es-CL,es_CO:es-CO,es_CR:es-CR,es_DO:es-DO,es_EC:es-EC,es_ES:es-ES,es_GT:es-GT,es_HN:es-HN,es_MX:es-MX,es_NI:es-NI,es_PA:es-PA,es_PE:es-PE,es_PR:es-PR,es_PY:es-PY,es_SV:es-SV,es_US:es-US,es_UY:es-UY,es_VE:es-VE,et_EE:et-EE,eu_ES:eu-ES,fa_IR:fa-IR,fi_FI:fi-FI,fil_PH:fil-PH,fo_FO:fo-FO,fr_BE:fr-BE,fr_CA:fr-CA,fr_CH:fr-CH,fr_FR:fr-FR,fr_LU:fr-LU,fr_MC:fr-MC,fy_NL:fy-NL,ga_IE:ga-IE,gd_GB:gd-GB,gl_ES:gl-ES,gsw_FR:gsw-FR,gu_IN:gu-IN,ha_NG:ha-NG,he_IL:he-IL,hi_IN:hi-IN,hr_BA:hr-BA,hr_HR:hr-HR,hsb_DE:hsb-DE,hu_HU:hu-HU,hy_AM:hy-AM,id_ID:id-ID,ig_NG:ig-NG,ii_CN:ii-CN,is_IS:is-IS,it_CH:it-CH,it_IT:it-IT,iu_CA:iu-CA,ja_JP:ja-JP,ka_GE:ka-GE,kk_KZ:kk-KZ,kl_GL:kl-GL,km_KH:km-KH,kn_IN:kn-IN,ko_KR:ko-KR,kok_IN:kok-IN,ky_KG:ky-KG,lb_LU:lb-LU,lo_LA:lo-LA,lt_LT:lt-LT,lv_LV:lv-LV,mi_NZ:mi-NZ,mk_MK:mk-MK,ml_IN:ml-IN,mn_CN:mn-CN,mn_MN:mn-MN,moh_CA:moh-CA,mr_IN:mr-IN,ms_BN:ms-BN,ms_MY:ms-MY,mt_MT:mt-MT,nb_NO:nb-NO,ne_NP:ne-NP,nl_BE:nl-BE,nl_NL:nl-NL,nn_NO:nn-NO,nso_ZA:nso-ZA,oc_FR:oc-FR,or_IN:or-IN,pa_IN:pa-IN,pl_PL:pl-PL,prs_AF:prs-AF,ps_AF:ps-AF,pt_BR:pt-BR,pt_PT:pt-PT,qut_GT:qut-GT,quz_BO:quz-BO,quz_EC:quz-EC,quz_PE:quz-PE,rm_CH:rm-CH,ro_RO:ro-RO,ru_RU:ru-RU,rw_RW:rw-RW,sa_IN:sa-IN,sah_RU:sah-RU,se_FI:se-FI,se_NO:se-NO,se_SE:se-SE,si_LK:si-LK,sk_SK:sk-SK,sl_SI:sl-SI,sma_NO:sma-NO,sma_SE:sma-SE,smj_NO:smj-NO,smj_SE:smj-SE,smn_FI:smn-FI,sms_FI:sms-FI,sq_AL:sq-AL,sr_BA:sr-BA,sr_CS:sr-CS,sr_ME:sr-ME,sr_RS:sr-RS,sv_FI:sv-FI,sv_SE:sv-SE,sw_KE:sw-KE,syr_SY:syr-SY,ta_IN:ta-IN,te_IN:te-IN,tg_TJ:tg-TJ,th_TH:th-TH,tk_TM:tk-TM,tn_ZA:tn-ZA,tr_TR:tr-TR,tt_RU:tt-RU,tzm_DZ:tzm-DZ,ug_CN:ug-CN,uk_UA:uk-UA,ur_PK:ur-PK,uz_UZ:uz-UZ,vi_VN:vi-VN,wo_SN:wo-SN,xh_ZA:xh-ZA,yo_NG:yo-NG,zh_CN:zh-CN,zh_HK:zh-HK,zh_MO:zh-MO,zh_SG:zh-SG,zh_TW:zh-TW,zu_ZA:zu-ZA
Now you can use xx-XX syntax on your local setup.
For reference, see my complete config file here:
http://www.nonumber.nl/files/config
If you need to move translations from xx to xx_XX groups, see:
http://opentranslators.org/en/forum/topic/70-developers-moving-translations-from-xx-to-xxxx-on-transifex