S̈, s̈ in lower case, also s with diaeresis, is a letter in the Chechen language, where it represents the voiceless postalveolar fricative. It has the same sound as the Slavic š, the Turkish ş and the Albanian "sh".
In the Chechen language, it was changed from the original ş to the s̈, at the same time the ç was changed into the c̈.
In older Czech orthography s̈ was used in codas instead of ſſ for /ʃ/, modern orthography uses š for all instances.
References
S̈ Wikipedia(Text) CC BY-SA